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1.0 Summary

I.I Introduction: The Advanced Solar Observatory (ASO) is a comprehensive long duration solar space

observatory which is planned to provide solar astronomers with the observational power (spectral. spatial, and

temporal resolution, sensitivity, and breadth of wavelength coverage) necessary to address fundamental problems

relating to the solar convection zone and activity cycle, the thermal and non-thermal processes which control the

transport of energy, mass and magnetic flux in the solar atmosphere, the generation of the solar wind and the

dynamics of the inner heliosphere. The ASO was recommended as the highest priority solar physics program for the

decade of the 1980's in the report of the Astronomy Survey Committee [The Field Report (1, 2)]. The Science

Working Group established as a result of the Field Report has developed an exciting concept for the evolutionary

development of the Advanced Solar Observatory O) which encompasses four instrument ensembles; (i) the Orbiting

Solar Laboratory [OSL (4)] that includes visible and ultraviolet telescopes, (ii) the High Resolution Telescope

Cluster [HRTC (3)] that includes extreme ultraviolet, XUV, soft x-ray and gamma ray telescopes, and Global

Dynamics Instrumentation, (iii) the Pinhole/occulter Facility [P/OF (5)] that includes Fourier transform and coded

aperture x-rayand gamma ray telescopes and occulted ultraviolet and visible light coronagraphs, and (iv) the High

Energy Facility _ (6)] that contains neutrou, gamma ray and low frequency radio spectrometers. OSL is currently

the highest priority moderate mission candidate within the Office of Space Science and Applications (OSSA). The

ASO Science Working Group (ASOSWG) has recommended the deployment of the remaining ASO instrument

ensembles on the Space Station. The ASO will operate as a facility open to the entire asu'onornical community.

Instrument concepts, which incorporate innovative new technologies, have been developed by the

ASOSWG and others, however detailed scientific analysis, comprehensive engineering studies, and an assessment of

the state of the relevant technologies critical to these updated instrument concepts is essential to the further

development of the ASO. Preliminary results of an ongoing accommodation study (7) have confirmed the

assessment of the ASOSWG that the ASO can be accommodated on the Space Station. However, in depth studies are

required to address critical accommodation issues, notably pointing, data handling, command and control. A major

goal of the concept study proposed herein is to carry out these deaailed analyses, studies and assessments.

1.2 Scientific Program of the ASO: The scientific objectives of the ASO are to address fundamental questions

relating to the Sun. in particular, and cool stars, in genezal; the specific objectives of the observatory arc:

i) tO provide instruments which are capable of observing solar phenomena on the inherent physical

scales of the underlying processes that control these phenomena. For the Sun this fundamental

physical scale, which is primarily that of the fine structure of the solar magnetic field, ix -70 km.

ii) to allow the Sun to be studied simultaneously utilizing the entire electromagnetic spectrum, from

high-energy gamma radiation to low-frequency radio waves. This capability will allow phenomena,

such as solar flares, which manifest themselves at all temperatures present in the solar atmosphere,

to be observed in their entirety.

iii) to provide the diagnostic capability necessary to determine the detailed physical conditions extant

within the solar plasma on spatial and temporal scales which will allow a direct comparison of our

present understanding of physical theory with the phenomenology displayed by the solar plasma.

1.3 Objectives and Significant Aspects of the Study: The objective of the concept study is to address the

following issues related to the development of the ASO and its deployment and operation on the Space Station:

definition of the requirements of the HRTC and HEF instrument ensembles to a level of detail sufficient to allow the selection of principal investigator teams for their development by the AO process.

(The definition of the requirements of the P/OF is the subject of a separate Concept Study.)

ii) definition of the facility equipment necessary to support the operation of the ASO on the Space

Station. The most important issues to be addressed pertain to the pointing, data handling, and command

and control requirements of the instrument ensembles.
iii) development of a plan for the operation of the ASO, including coordinating observations of the four

ASO instrument ensembles, insuring access to ASO by the astronomical community, defining the

operational modes of the ASO, and the distribution and archiving of data.

iv) development of a plan for the implementation of the ASO program, including management plans for

the evolutionary construction and deployment of the ASO instrumentation, and for the management of

the necessary ASO scientific and support operations. The evolution of ASO capabilities, which is

essential for a long duration observing facility, will also be considered.

1.4 Investigative Approach: Our approach will be to emphasize detailed studies of the observational and

operational requirements the ASO instruments must meet in order to address the major scientific questions we have

posed, and the evolutionary development which the ASO must be capable of in order to meet new and as yet

unforeseen requh'ements which will develop as the major scientific issues facing solar physics evolve. The methods

we plan to use are the following:

i)

ii)

During the initial phases of the study, we will conduct a series of interactive studies of the design and

operation of each ASO instrument. The design studies will be led by active solar instrumentalists and

observers who will be designated as Instrument Scientists, with the assistance of consultants (who are

also active members of the solar community) having expertise in areas which are critical to the design

studies, such as detector technology and optics technology. The operational studies will be led by

theoretical astronomers and observers, who will be designated as Mission Scientists. Subsequendy, the

Instrument Scientists and Mission Scientists will lead a series of scientific symposia which will

provide a forum for the entire solar community to participate in the definition of observational

capabilities and operational modes of the ASO instruments.

Based on the insu'ument design and operation studies and scientific symposia described alive (and those

anticipated from the parallel P/OF.study mentioned earlier), we will further refine the accommodation

requirements for the deployment of the ASO. This will allow the formulation of a specific set of issues

pertaining to the accommodation of ASO on Space Station. We plan to initiate a number of

subcontracts to address these specific issues. The issues we are able to identify now are (a) engineering

.a_. yses of the strawman Ago instrument concepts, Co) a study of alternative strategies (i.e., a fine

pointing platform mounted on the Space Station Pointer, articulation of optical elements within the

individual insu'uments, image motion compensation within the detectors, etc.) to meet the pointing

requirements of the ASO instruments on Space Station, (c) a study of alternative su'ategies (i.e., on

orbit data compression and analysis, transport of optical disks and photographic film via the Shuttle,

teleoperation from the ground, control by mission specialists, etc.) to meet the operational command,

control and data requirements of ASO on Space Station, and (d) a continuation of studies of the

accommodation of ASO on Space Station, as the Space Station characteristics are defined in more

detail.

iii) We plan to develop a detailed plan for the operation of the ASO. Specific issues which must be

addressed include the coordination of observational programs for the ASO instrument ensembles on

different platforms, selection and implementation of scientific investigations for the ASO. control of

observing sequences, handling and archiving of data, etc. The methods we plan to utilize include

simulation of ASO observing sequences designed to address specific scientific issues of current interest.

in scientific seminars to be led by the Mission Scientists.

iv) We plan to investigate the approach used to manage the development and operation of other major

NASA (ST, ORO, AXAF. SIRTF) and NSF (NOAO, NRAO) observatories as the initial step in the

development of a plan to manage the development and operation of the ASO. Prs of major NASA

programs, and directors of major observatories will be consulted. Based on these discussions, we plan

to develop a complete management plan for the ASO, including coordination of evolutionary

development of the ASO instruments, development of instrument support equipment (both flight

equipment and ground-based equipment), operation of the ASO, and distribution and archiving of data.
1.5 Instrumentation: Table l presents an abbreviated summary of the anticipated performance of the ASO

strawman instrument complement. Since the final ASO instrument complement will be selected by AO, our goal in

the present study will be to model specific instrument designs to demonstrate that the performance goals that we

have established can be met, and to identify the technologies that are critical to meeting those goals. Generalized

specifications for the integration, and pre- and post-launch operation of ASO instruments on Space Station will be

based on the detailed study of specific instrument designs. Specifications for the ground operations necessary to

support the operation of the ASO on Space Station as a national facility will also be developed.

1.6 Data Acquisition Calibration Reduction, Analysis, and Archiving:. It is probable that the acquisition of

data by the A$O will utilize a number of media, including photoelectric detector arrays, photographic film. and

storage on optical disks. The volume of data will present a formidable challenge. In order to address the issues raised

by data acquisition, calibration, reduction, analysis and archiving, we will develop detailed models, of typical data

sets that will be necessary to address specific problems of current interest by the use of simulations of observational

campaigns. These simulations will be led by the Insttmnent Scientists and Mission Scientists. and will include a

significant number of additional scientists from the solar community.

1.7 Role of the Crew and Payload Specialists: The interaction of the crew and payload specialists with solar

instruments on Skylab. and on Spacelab I and 2 was an important component of the success of these missions.

Definition of the role of the crew and payload specialists will be based on the past experience of major solar

observatories on manned missions, and on the detailed instrument definitions and mission simulations to be carried

out during the course of the study.

1.8 Cost and Management Plans: A major goal of the ASO concept study will be the development of a detailed

management plan and cost model for the development, deployment and operation of the ASO facility.

TalDte 1

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ASO m [ammt_m

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D Our suawnummnl_;W_lNnm_smns 3 XUV_ mm mecIml owemlle IS0.110 A. 190. Zl0 _. and;MO. 310A

f Entnes _*r 0oh*ghnmiuaon am Iq_ s*,Mmvsymeommews nmmm,_/ Emrm r_ m_ nJmmuImx_o w_ _ mogol _

g Fq_efaengtot_n_tm_Ugmmom. (3morm_mmlf_muwremluim.

High Rmmk_on Tekmcqae Clustff

Soa X-'Ft.' TWucqau a 13-170 J_ 40¢m 0.1S/0.4 f 10.000 0.01 _5. x 3.S'n2. x 124

XUV Tmgol_ "b 150 - 310 A 40 an 0.1/0A f 20.000 0.01 3.5' • 3.5'/12" • 124

ELN TilleScope 550 * 1100 _ 40 ¢lli 0.1/0.4 f 30.000 0.1 3.5' • 3.5"/12"• 124

Gamma Ray muqng 13mm_ 2 - 1000 keV 50 an 1.0 10 0.001 lun sm

X-Ray Fi,lm Specm)mm_ 1.5 - 2S_ 40 an 1.0 10.000 0.01 lul sun

GiooaJOynanU:s _ 3500 - 11.000 A 50 an 0.fu'S= I00.0004: 1.0 lul sun

I.mrav_m Te*escq_ 117S- 1700 A 60 an 0.1 30,000 0.1 3.5" • 3.5"

High Enemy Fa_my

Gamma RaY LJn4SPeClmrnm[Ms4 10 keV-10MeV 30 ¢_InYS0¢m - 400/20 1/0.25 lull sun

I-i_ Erer_ Galnma P.W Spe¢_mnet_ 10 MeV-100 MeV 100¢m - S 0.001 lul sun

l.lgh Energy Neufron _er 10 MeY-1000 k4eV 100 (:m - $ 0.001 lul sun

Low FN_luency Radio S01¢ln)grll_ I *20 _ 30.000 S0 200 1 lull lath

Plnhole/Occulter Fadllff

CoOed Apem_ In'.,_er 2 - 70 key 100 em 4.0 10 0.001 lull sun

Fourier"rr'ans_orm_ 2 - 1000 key 100 cm 0.2 10 0.001 lull sun

Wh_e l.J_ Con_ 1100 - 11,000 _ 50cm 1.0 S.O00 1.0 lull sun

EUV Comnagraph 300 - 17100A 50c:m 1.0 20.000 0.1 lull sun

Oronm0 Solar Laboratory

Oplgal Te_ef,co_ 2000 - 11.000 _ 100¢m 0.1 100.000 0.1 1-3"x 1.3"/'J'x 3"

Ulravio_t Telesoope 117S- 1700 A 30cm 0.S 30.000 0.1 4'x4"
2.0 Scientific Objectives of the ASO

2.1 Scientific Motivation: The study oftbe Sun is an exciting and rewarding endeavor for several reasons.

• First, the Sun is the largest and most complex physical system which we can study in detail

Much of the Sun's complexity arises from the interaction between plasma and magnetic fields, as is evident

from the images on the cover and in the frontispiece. The diverse phenomena observable on the Sun provide an

important source of inspiration for the development of physical theory and a unique environment for its testing and

application, both at the microscopic level of atomic and nuclear processes and at the macroscopic level of the

collective behavior of matter in the presence of gravitational and electromagnetic fields.

• Second, the Sun occupies a central position in astrophysics.

Because of its proximity, it is the only star which can be examined in sufficient detail to allow

astrophysical phenomena to be studied at the level of the basic plasma physics and magnetohydrodynamic processes

which underlie these phenomena. The Sun is, in effect, a laboratory which we can utilize to study the physics of

stars and other astronomical objects.

• Third, the Sun generates and modulates the interplanetary medium.

The volume of space dominated by the solar wind is now generally referred to as the heliosphere to

emphasize the primary role of the Sun. The solar wind, and its interaction with planetary magnetospheres,

ionospheres, and atmospheres, is the subject of the discipline of space physics; the connection between solar physics

and space physics is no less profound than is the connection between solar physics and astrophysics.

• Finally, the Sun has a profound influence on the Earth's ecosphere, and hence on humanity.

Life on Earth is possible only because of the Sun, and our lives depend on the use of solar energy, either stored

or ambient. Moreover, in the past several years; evidence has steadily mounted that the variability of the Sun, which

is a consequence.of the Sun's magnetism, can significantly affect conditions on the Earth on time scales of centuries

and millennia, and perhaps on shorter time scales as well. Our quest to understand the Sun's magnetic variability,

which is represented by (but by no means limited to) the well known sunspot cycle, is therefore highly relevant to

the terrestrial sciences of meteorology and climatology.

Because the solar magnetic field in and above the photosphere is strongly inhomogeneous on all scales down to

well below an arcsecond, to significantly advance our kntwledge and understanding of solar magnetic structure and

activity, we must observe all levels of the solar atmosphere, from the photosphere to the corona, with subarcsecond

resolution. This requires a set of large aperture telescopes placed above the atmosphere and observing in coordination

to achieve the necessary spatial resolution across the spectral range that corresponds to the range of temperatures

present in the solar atmosphere, i.e., from visible light (photosphere and chromosphere) to X-rays and gamma rays

(corona). The Advanced Solar Observatory on the Space Station will realize this goal. The cluster of solar telescopes

on Skylab, although not as comprehensive as those proposed for the ASO, demonstrated the power of such

coordinated observations. The Skylab telescopes, with resolution of a few arcseconds, showed us that the solar

magnetic field dominates the structure and behavior of the solar atmosphere: the Advanced Solar Observatory on

Space station, with subarcsecond resolution, will help reveal the physical processes responsible for these structures

and phenomena.

2.2 A Brief Review of Recent Advances in Solar Physics: In the past 15 years, observations of the Sun from

space and from the ground have yielded discoveries which have greatly enhanced our knowledge of solar phenomena

and of their connections to the other disciplines cited above. Among the most significant of these discoveries are:

• The detection of neutrinos from the Sun (8), the first direct experimental confirmation of the central

role played by thermonuclear processes in stars.
Theobservedneutrinofluxisafactorof 3 lower than that predicted by standard solar models. The cause of

this disagreement between observation and theory remains an unsolved mystery.

• The discovery that the 5-minute oscillations of the Sun are a global seismic phenomenon that can be

used as a probe of the structure and dynamical behavior of the solar interior (9).

Precise measurements of these oscillations provide a new window on the structure, transport of energy and

generation of magnetic fields in a star's convective envelope.

• The discovery that the damping of compression waves driven by subphotospheric turbulent convection

generates too little energy to heat the corona and drive the solar wind (10).

The correlation between the observed x-ray luminosity of other stars (that, like the Sun. have convective

envelopes) and their rotation rate has reinforced the conclusion (drawn from solar observations) that magnetic effects

underlie many of the active phenomena observed in stellar atmospheres.

• The discovery that, when viewed on a fine scale, the solar magnetic filed is rooted in discrete flux tubes

with field strengths exceeding 1000 gauss (U), and diameters of approximately 100 km (~0.1 arc see).

The physical size of these fundamental magnetic flux elements is too small to be resolved by any present

telescope. Why the solar magnetic field should be structured in this way is not understood.

The demonstration that the large scale solar magnetic filed is organized into two distinct types of

structures; magnetically closed regions, in which hot plasma magnetically confined in loops largely

generates the x-ray corona; and magnetically open regions, the so-called "coronal holes, _' which are

the source of high speed streams in the solar wind (Figure 1) (12).

• The confirmation of the evidence (provided by 17th century observations) that the sunspot cycle and

associated active phenomena were largely absent for a period of 70 years in the 17th century.

This episode is known as the Maunder Minimum (9,13). There is evidence that such interruptions, along

with periods of heightened activity, occur quasi-periodically and that there is strong correlation between the general

level of solar activity and the the occtaxeor_ of climatic changes on the Earth. The cause of this modulation of the

solar activity level is unknown.

The recognition, as a result of observations of hard x-rays, gamma rays, and energetic neutrons, that

the energy released during the impulsive phase of a flare is initially largely, or entirely, contained in

non-thermal particles accelerated during magnetic-reconnection processes in the coronal field (14).

• The discovery that the ejection of large clouds of gas called coronal mass transients (8) (such as those

shown in the frontispiece) can accompany some flares.

The profound impact of these and other discoveries on our appreciation of the complexity and diversity of

solar phenomena has led to the maturing of solar physics as a scientific discipline. This new maturity has allowed

solar physicists to formulate a much more wecise theoretical and observational strategy for their discipline (15). The

centerpiece of the observational component of this new strategy will be the Advanced Solar Observatory.

A comprehensive review of the current status of solar physics is contained in the three volume set "The

Physics of the Sun" (16) which is recommended to the reader who wishes to pursue in depth any of the specific

topics reviewed above.

2.3 A Scientific Strategy for Solar Physics: The report of the Solar Physics Working Group of the Astronomy

Survey Committee (15) has recommended three themes or areas of concentration as potentially the most productive

for solar physics over the next decade. These three themes are: (i) the development of observational techniques

capable of probing the interior structure, dynamics, and composition of thc Sun; (ii) the study of the "active
Figure 2 A wide-bend (60 _) image of the solar granulation ccmercxl on the

calcium K line ;L3934 _,). The bright gramdalio, cclls, npl)ro._imalcly I -2

arcseconds (700 - 1400 kilometers) in dia.,eter, are the Celmlcrs of .l_ward

moving convection cells which briqg hal material from the I(_wcr

photosphere Io Iho surface. The d:lrk houndarics conl;,in cooler matcri:tl

which is flowing dew.ward.

Figpre I "[ltis x-ray photograph of Ihe Sun, lakcn Aqgust 21, 1973, wilh

the American Science anti Engineering Jnstrmnen! on Skylab, shows large

coronal loop struclures and many small brighl points though Io bc loops

that are too small to be resolved. A largo corunai hole cxtcnding From the

north pole across the eqtmtor is plainly visible.

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Figure 3 Narrow band images of the solar atmosphere obtained near the

center of the magnesium BI line _.5184 _). In the photograph at the left

(+0.4 _, from the line core) which corresponds to higher levels in the

atmosphere, the snporgranulation cell boundaries me visible as a bright

network with a cell size of approximately 30 areaeconds (20,000

kilometers). The solar granulation is clearly visible in the image at the right

taken in the wing of the line (+0.8 A).

Figure 4a The power distribution over the normal modes of global

oscillations of the solar surface. The vertical and horizontal axes are the

temporal and _patial frequency over Ihe range._0 - 0.5 cycle_mm a_KI 0 -316

cycle,Vsolar diameter;, the brightness corresponds to tile mul_lilu(Ic of the

oscillalions. The concentration of the power along Ihcse discrete ridges

shows that the oscillations ,'ire the nouradial global modes in which i)rc._surc

is the main resloring force.

I-2. m-O I- 1O, m-0 1-10.m-$0 I-Sl, m-S0

Figure 5 Montage of the magnetic structure of Ihe quiet and active Sun. On

the left is a magnetogmm of half of the solar disk taken at a quiet time near

minimum in the magnetic activity cycle when no major active regions were

present. The right half is a magnctogram taken near the activity maximum;

it shows the complex distribution of strong magnetic flux in the sunspot

7.one._ntwth and sough of the equator.

Figure 4b Oscillations of the Sun's surface are Ihe result of sound waves

resonating in its interior; here 4 of the ten million resonances thai Occur are

modeled by computer. Surface regions that are approaching the obscrver are

colored blue; regions that are receding are colored red. Sllch surface

displacements are detected as Doppler shifL_ in the wavelenglhs of light

absorbed by the moving gases.

OR!G_',IAL PAGE

BLACK AND _,t/'-lllE PI-;OTOGRAPI-I
o

phenomena" such as flares, sunspots, the activity cycle, the chromosphere, the corona, and the solar wind, which are

a consequence of solar magnetic variability; and (iii) the study of the role of the Sun in shaping the threedimensional suucmre and dynamics of the hefiosphere. The last two themes will have important implications for our

understanding of the Earth's space environment and climate 07). We can formulate a coherent scientific program

which addresses these themes as a series of seven fundamental questions, which present an overview of the theoretical

and observational issues which will be the focus of solar research over the next decade.

I. What are the fundamental properties of the solar core (where energy is generated) and the radiative

interior (through which energy is transported to the Sun's outer layers)? In particular, what is the Sun's

internal rotation rate. chemical composition, and temperature distribution, and what is the detailed

process of nuclear energy generation and energy transport? How do these properties relate to current

theories of stellar evolution?

.

.

What is the magnetohydrodynamic structure of the solar convection zone. and what is the role of the three

convective scales observed on the Sun. the granulation (Figure 2), the supergranulation (Figure 3), and

the large scale circulation in transporting energy from the solar interior to the solar surface? Can a

generalized theory of stellar convection in the presence of rotation and magnetic fields be developed

which describes the structure of the Sun's convective zone and predicts the observed convective scales?

What physical mechanisms drive the solar magnetic field and activity cycle; what resulting variations in

the solar radiative and particulate output follow on various time-scales; and what is the effect of this

variability on the Earth's atmosphere, ionosphere, and magnetosphere? What causes the long-term

variations in the solar magnetic and activity cycles, such as occurred during the Maunder Minimum?

How do these phenomena relate to activity and variability on other stars?

. What processes, involving small scale velocity and magnetic files and various wave modes, determine the

thermal structure and dynamics of the solar photosphere, chromosphere, and corona, and what are the

implications of such processes for stellar atmospheres in general?

. What are the basic plasma-physics processes responsible for metastable energy storage, magnetic

reconnection, particle acceleration, and energy deposition in solar flares and related non-thermal

phenomena? What are the implications of these for other high energy processes in the Universe?

.

,

What "ate :the large scale structure and plasma dynamics of the solar corona, including the processes

involved in heating various coronal structures and initiating the solar wind? What are the implications for

stellar coronae and winds other ustrophysical flows? What is the origin of coronal transients?

What are the implications of coronal structure for the three-dimensional structure and dynamics of the

heliosphere and what are its implications for cosmic ray modulation and for the modulation of planetary

atmospheres, ionospheres, and magnetospheres, including those of Earth?

The broad scientific objectives of the Advanced Solar Observatory are those of the above fundamental questions.

These objectives will be the focus of solar physics into the 21st century. The Advanced Solar Observatory will

provide the new generation of improved observations needed to penetrate the entire range of problems defined by the

above questions, viz., the structure and dynamics of the convection zone, the nature of solar magnetism and the

activity cycle, the structure and dynamics of the chromosphere and corona, the nature of flares and other nonthermal

phenomena, the generation of the solar wind, and the structure and dynamics of the Hcliosphcre. In addition,

although the study of the composition, structure, energetics, and dynamics of the solar core are the focus of a number

of other NASA and NSF programs, the Advanced Solar Observatory, by virtue of its ability to study solar

oscillations, can make significant contributions there as well.

2.4 Specific Objectives of the Advanced Solar Observatory:

2.4.1 The Structure and Dynamics of the Convective Zone: In the Sun's convective envelope, radial free

convection in the presence of the rotation of the Sun is thought to give rise to differential rotation and other global

flows. These global flows, perhaps in combination with smaller-scale convective motions, are the likely drivers of

the dynamo process which generates the magnetic flux that emerges from the convection zone into the solar
atmosphere to produce all of the manifestations of the magnetic activity, e.g., sunspots, active regions, flares,

chromospheric and coronal structure and heating, and the global behavior of the emergence and dispersal of the

magnetic field. All known variations of the sun's output of radiation, particles, and magnetic flux into interplanetary

space, and hence the terrestrial effect of these variation, are consequences of the sun's magnetic activity cycle.

Obviously. understanding the dynamical processes in the convection zone is essential to the study of the extended

solar atmosphere and its dynamics.

In the last decade, a new technique has been developed which can be used to probe the structure,

composition, and dynamical behavior of the solar interior, which is, of course, not directly observable. Helloseismology involves the analysis of the radial and nonradial oscillations (Figures 4a and 4b) of the Sun from which

the variation with depth of the sound speed in the Sun's interior can be determined; from the sound speed, the

structure, composition, and dynamics of the solar interior can be inferred. Together with studies of the three scales of

convective motions on the Sun, [the granulation (Figure 2), the supergranulation (Figure 3). and large scale flows.

(by precise studies of the velocity and brighmess pattern of the photosphere)], helioseismology should allow us to

address the following basic questions.

Are theregiantconvectivecellson scalesbetweenthatofsupergranules,and thatoftheglobal

meridionalcirculation?

• How does the Sun's internal rotation rate vary with depth and latitude?

Do the internal rotation or the cellular structure of the convective zone change systematically each

solar cycle?

How does the Sun's temperature vary with depth and latitude? Is this thermal structure timedependent?

How are changes in the strength and location of magnetic structures within the convection zone

correlated with changes (e.g.. by flux emergence) in the magnetic structure of the solar atmosphere?

What is the extent and impact of turbulent phenomena (e.g. turbulent diffusion) on the convection

zone.?

2.4.2 Active Phenomena on the Sun: Active phenomena on the Sun are the result of the variability of the solar

magnetic field, which is a consequence of the energy transport processes operating ir_the convection zone (9a). We

divide solar activity into four aspects: (i) magnetism and mass motion, including the activity cycle; (ii) atmospheric

heating and mass transport, including the structure and dynamics of the chromosphere and corona; (iii) flares and

particle acceleration; and (iv) the generation of the solar wind.

2.4.2.1 Solar Magnetism and the Activity Cycle: One of the major characteristics of the Sun (and of many other

stars) (18) is a magnetic field which varies cyclically (Figure 5). The cycle period for the Sun is about 11 years

without regard to magnetic polarity or 22 years taking account of polarity. There is evidence that the strength of the

cycle may at times be substantially weaker or stronger than recent cycles (Figure 6). However, a remarkable analysis

of 1760 years of annual sediment deposits in an ancient glacial lake provides evidence that the solar cycle was present

680 million years ago with the same period as now. Of great practical importance to mankind is the fact that the

enduring magnetic activity cycle (9a) is the source of the variation of the solar radiative and particulate flux.

The currently prevailing (but not universally held) view of the basic dynamo processes that sustain the solar

cycle invokes amplification of a relatively weak magnetic field by shearing motions at the base of the convection

zone. This action forms buoyant flux bundles which rise fairly rapidly through the convection zone and erupt at the

visible surface to produce active regions. Frequently a substantial amount of energy is released at the eruption site in

the form of flares and other active Sun phenomena. The total radiative output of the Sun is also disturbed by these

magnetic eruptions. Most of the magnetic flux disappears rather rapidly near the site of the eruption. The remaining

flux spreads across the surface of the Sun (Figure 7) by the combined action of the granulation, supergranulation,

differential rotation, and meridional flow. During this phase, the flux exists mainly in the form of small, isolated
• m

= ,,.

v|_ll

Figure 6 The apparent correlation of solar activity and climate: the lower

curve, ba.,ed on tree-ring dam, represents the rate of production of carbon-14

by cosmic ray bombardment of the upper atmosphere. This production'

varies inversely with solar activity. Plotted above am measures of mean

European climate: the advance and retreat of Alpine glaciers, historical

inferences of mean annual temperature, and the recorded savorily of northem

European winters. The temporal coincidence of low solar activity ,'rodc(xd

European climate suggests a cau.,ml connection between long-term solar

behavior and climate, although other data indicate a more complex

relationship.

Figure 7 A high resolution magnetogrmn of a complex of active regions, [t

shows that the photospheric magnetic flux in and bcyon(I the outskirts of

strong active regions is eonccnlrated at the bolmdarics of supergranulation

convection cells, and the the field has strong line sonic suucturc down to the

limit of resolution (2 arcsec).

Figure 8 Spicules are thought to be tubes of magnetic flux containing

upwardly moving gas. This offband H-a photograph of the solar limb

emphasizesthese spike-like structure& Spicules appear at the boundaries of

supergranular cells. The magnetic field is concentrated in the supergrenule

boundaries, which underlie a network of hal. material with temperatures

ranging from I0,000 to 700,000 Kelvin.

Figure 10 Solar flare gamma ray spectrum obtained by the Solar

Maximum Mission, showing strong nuclear gamma ray lines. This

spectrum, shown hem with background and flare brcmsslr:lhhmg subtracted,

was pr(xltlCC(I by the limb flare on April 27, 1981, which I_gan at 08;05

UT. The numbered peaks arc emission lines from the excited nuclear Stales

Of the isotopes Oxygen-16, carbon-12, nitrogcn-14, dcutcrhnn, neon-20,

iron-56, magncsium-24, beryllium-7, and lithimn-7 among others, and from

positron-electron annihilation. Many of the features are strongly Doppler

broadened. (Courtesy of Ed Chupp, University of New Hampshire.)

BLACK

Figure 9 Complex of coronal loops at the solar limb, photogralphcd in the

light of a resonance llne of C IV (-150,000 Kelvin) at _,1548 A. with the

UIIraviolet Spectrometer and Polarimulcr on the Solar Maximum Mission.

The pholograph at the left shows the inlenaity, while the p_otograph at the

right, which was obtained from the differen(_ in intensity of the tad and blue

wings of the line, represents the velocity distribution within the loop.

Material is apparently flowing in the loop, emphasizing the dynamic n:ltUre

of the apparendy static features in the solar atmosphere.

April 27, 1981

800 • ..... I ...... v ...... _...... r ...... _......

®

600

4oo

0

2O

2OO

®@

160 300 440

Channel number

ORtG;NAL PAGE

AND WHITE PHOTOGRAPH
vertical bundles having a large flux density and low gas density. A substantial amount of this flux gradually

disappears from our observations. The small amount of surviving flux eventually concentrates at the poles where as

yet uncertain processes may operate to seed the beginnings of a new cycle.

While many of the observed aspects of the magnetic cycle have been reproduced in kinematic dynamo

models, serious doubts have been raised recendy about the validity of these models. What is required is a theory of

the dynamical behavior of the convection zone, based on the fundamental physical principles, which can predict the

convective and magnetic properties of the Sun's outer layers. The acquisition of an observational record which can

guide astrophysicists in the development of such a theory and provide the observational tests required to refine the

theory, is a fundamental objective of the Advanced Solar Observatory.

Among the basic questions relating to the activity cycle and the magnetic field which will be addressed by

the Advanced Solar Observatory are:

What are the factors which control the modulation of the activity cycle (Figure 6)?

!.

What is the fine scale structure of the solar magnetic field, and what factors control this structure?

What factors control the generation, transport, and dissipation of magnetic flux?

How does the magnetic field modify the flow of energy in the outer layers of the convection :one and

in the solar atmosphere?

2.4.2.2 Atmospheric Heating and Mass Transport: The occurrence of a temperature reversal in the solar

chromosphere, the million-degree temperatures characteristic of the corona, and the expansion of the corona into the

solar win.d all demonstrate that conditions in the solar atmosphere are far removed from local thermodynamic

equilibrium (LTE). Non-thermal phenomena, which include the propagation and dissipation of waves, the generation

and dissipation of macroscopic flows and coronal current systems, and the confinement of the plasma by magnetic

forces, have a major influence on the structure, dynamics, and energy balance of the atmosphere.

Although the photosphere and chromosphere are layers of almost negligible geometrical thickness compared

with the radius of the Sun (somewhat analogous to the outer skin of an onion), they ate of fundamental importance

to the energy balance, mass balance, and composition of the entire outer solar atmosphere, including the solar wind.

The energy and. m_s flux that ultimately produce the corona and solar wind move through these lower atmospheric

regions, leaving their signatures there; the energy dissipated in the photosphere and chromosphere is roughly 10

times greater than that required to heat the entire corona and solar wind. The photosphere is highly structured by

convective and magnetic processes and is highly dynamic, with the granulation structure (Figure 2) constantly

changing. The complexity of the structures which control the transport of mass and energy in the

chromosphere/corona interface is illustrated by Figure 8, which shows the upward flowing columns of gas called

spicules, which are confined by magnetic flux concentrated at the boundaries of the supergranulation cells. The

chromospheric material is concentrated in a network of hot low-density gas, which traces out the supergranulation

boundaries. This network is visible in the mottled appearance of the solar disk in the He II (304 A) image in the

frontispiece, and in EUV lines emitted at temperatures up to 7 x 105 K (cover image).

While the magnetic field is an important factor in the dynamical behavior of the chromosphere, it is the

dominant factor in the corona. High resolution observations (such as that on the cover) have demonstrated that the

coronal layers of active regions (within 1 to 2 solar radii of the surface) consist primarily of elemental structures in

the form of magnetic flux tubes or loops in closed configurations (i.e. with both footpoints rooted in the

chromosphere) which contain material ranging in temperature up to 10,000,000 Kelvin or more. One of the surprises

of the 1970's was the discovery of cool loops, filled with plasma at temperatures between 100,000 K and 1,000,000

K. These loops appear to be dynamically and thermally unstable, with material flowing downward into the

chromosphere (Figure 9). The exchange of mass and energy between the chromospheric and coronal levels in these

structures appears to play a major role in determining the temperature-density structure of the coronal layers of active

regions.

The ASO will provide the observational data required to guide the development of theoretical models of the

structures and processes that control the transport and dissipation of energy and the flow of material in the

photosphere, chromosphere, and inner corona, and to provide the observational tests required to refine these models.

These observations will address the following fundamental issues:
What is the detailed structure and energy balance of the temperature minimum region in the lower

chromosphere, where the rise to coronal temperature begins?

What processes control the non-LTE transport of radiation through the chromosphere, and what is

the effect of the atmospheric fine structure and differential velocity fields on these transport

processes?

Although acoustic waves have been ruled out as the principal agent responsible for the heating of the

corona, they may still play an important role in the chromosphere. Among the questions which must

be addressed in connection with the role of acoustic waves are: (i) how does the dynamic behavior or

the solar granulation generate acoustic waves, and what are the spectral characteristics and energy

flux of these waves; and (ii) how are acoustic waves dissipated in the chromosphere?

• What physical process cause the rising and surge-like behavior of spicules?

What are the roles of the conduction of heat from the corona and direct magnetic heating (via current

dissipation) in maintaining the thermal structure of the chromosphere and transition region?

What is the relative role of the dissipation of mognetohydrodynamic waves and of coronal current

_stem$ in the heating or coronal loops? Do rapid reconnection processes play a significant role in

heating the "quiet" corona, or are they only important in flares? What roles do thermal conduction

and macroscopic mass flow play in th energy balance of coronal loops?

What is the role of the newly emerging regions of magnetic flux which manifest tl_emselves by

enhanced EUV and soft x-ray emission (the bright points) in the magnetic evolution of the solar

atmosphere and leakage of ma_ into the solar wind?

Although theoretical diffusion models have predicted composition differences between the

chromosphere and corona, no such differences have been observed. What processes cause the

atmo. spheric miMng which eliminates these composition differences, and do occasional interruptions

in these processes contribute to the composition anomalies observed in the solar wind?

What characterizes the flows in the chromosphere that preserve mass balance in the corona, while

providing leakage to the solar wind, and how does this affect the energy balance of the

chromosphere?

2.4.2.3 Flares and Particle Acceleration: The basic flare mechanism is an explosive conversion of magnetic

energy into high energy particles ('Figure 10), bulk mass motion (frontispiece), and heated plasma. Since the

fundamental ingredient is magnetized plasma in which energy has been stored in the magnetic field through nonuniform motion of this medium, and since dynamic magnetoplasma systems in and around stars and galaxies pervade

the Universe, flares are of general astrophysical importance. In solar flares, both the buildup and the release of energy

appear to depend critically on the fine scale (less than 1000 kilometers) structure of the magnetic field, and involve

all levels of the solar atmosphere. All flares drastically affect the low corona and chromosphere; large flares can

strongly affect the outer corona and solar wind, and sometimes noticeably heat the photosphere. The accelerated

panicles range in energy from at least 10 keV to more than I GeV (I GeV equals 1000 MeV), the heated plasma

ranges in temperatures from photospheric (5,000 K) to more than 50,000,000 K. Thus, to observe the overall flare

phenomenon and to identify the energy release mechanism will require the full capability of the Advanced Solar

Observatory for coordinated high resolution observations spanning the pre-flare configuration of the solar atmosphere

and the full range of flare outputs.

The flare phenomenon can be divided into five distinct phases. In approximate order of occurrence, these are

energy storage, initiation of energy release, mass ejection and particle acceleration, energy transport and heating, and

atmospheric restructuring. Among the basic questions which must be addressed by the Advanced Solar Observatory

in order to develop a fundamental theoretical understanding of flares and the related non-thermal phenomena are:
What is the detailedstructureof the magnetic configurationsand currentsystems responsiblefor

storageof theflareenergy,and how do theseconfigurationsevolve?

What changes occur in coronal currentsystems inunediatelybeforea flare,and what isthe roleof

thefilaments of cool chromospheric ri_aterialwhich are observed in the low corona before many

flares?

What isthesiteof theenergyreleaseinflaresthatare observed tooccur in coronalloops?

In both filament eruption flares and coronal magnetic.loop flares, the initial release mechanism is

thought to be magnetic reconnection or tearing, which is the only known way of releasing the stored

energy rapidly enough. What are the details of this reconnection process, and how does it mediate

the impulsive transfer of energy from the magnetic fields to a populatio n of relativistic electrons and

ions?

What is the mechanism which, in many flares, causes the large scale ejection of mass (filament

eruptions) into the corona?

Does the presence of high-density currents during the reconnection process generate turbulence in

the solar plasma? If so, does the turbulence inhibit the escape of non-thermal particles from the flare

site?

How is the energy released by the flare initially transported to the surrounding solar .plasma,and

what is its impact?

What is the relative magnitude of the energy released in the impulsive reconnection process and the

energy that is released more gradually after the impulsive phase?

i

2.4.2.4 The Generation of the Solar Wind: Interest in the solar wind acceleration region stems from the

astrophysical problem of stellar winds (and expanding hydromagnetic flows from stellar systems) and the space

physics problems of the heliosphere and solar-planetary relations, as well as from the influence of this region on the

energy and mass balance of the corona. Most of the acceleration of the solar wind is believed to take place within 10

solar radii of the Sun's center. It is in this region that the magnetic field changes from being mostly closed to being

mostly open. Over this radial span, the coronal plasma changes from being collision-dominated to being nearly

collisionless: as a result, the charge state of coronal ions entering the solar wind is expected to be frozen in the

acceleration region. Variations in the chemical composition of the solar wind, measured near the Earth, may be

produced here also. Because of the long thermalization times due to the low densities, non-thermal processes (e.g.,

magnetohydromagnetic waves) may produce detectable spectroscopic signatures for particles of different mass; this

would help identify mechanisms of plasma heating and momentum deposition. In addition, flares and filament

eruptions can produce coronal mass ejections (frontispiece) that disturb the corona and solar wind strongly enough to

dramatically affect the Earth's magnetosphere.

The ASO will provide unprecedented capabilities for observing the generation of the solar wind and for

relating structures and events in the acceleration region to their origins in the lower atmosphere. Specific issues

which must be addressed in the study of the solar wind acceleration region are:

What are the processes that deposit energy and momentum in the outer corona to generate the solar

wind?

• What are the processes that heat the corona in closed magnetic regions?

What are the mechanisms by which the corona evolvesin responseto slowlyvaryingconditionson

the solarsurface?
What are the processes governing the expulsion of coronal mass ejections? ilow do they affect the

solar wind?

What ix the role of the solar wind. and of coronal mass transients in the overall angular momentum

lass and spindown of the Sun?

• How are the high and low speed streams observed in the solar wind generated?

2.4.3 The Structure and Dynamics of the Heliosphere: The heliosphere is that region of space in which the

physical conditions are established and controlled by the presence of the Sun; as such, it represents the largest scale

organization associated with our star. Our knowledge of the heliospbere is quite limited. We don no know its extent,

its detailed morphology or its evolution, although we are well aware of many features of its innermost regions, the

corona and the interplanetary medium in the vicinity of the Earth. Since the flow of the solar wind is governed by

conditions at the base of the corona, we expect that detailed knowledge of the corona will allow us to infer the

conditions throughout the heliosphere. However, internal dynamical processes, together with the results of influences

from without the heliosphere, will modify these conditions, perhaps significantly near the outermost boundary.

Thus, careful and coordinated studies of coronal and other direct indicators of the condition of the heliosphere and its

interface with the interstellar medium are required for an understanding of the physics of the heliosphere. The role of

the Advanced Solar Observatory will be to probe the physics of the coronal phenomena which directly structure and

disturb the interplanetary stretch of the heliosphere. Among the issues pertaining to the heliosphere which the

Advanced Solar Observatory will address are:

How is the three-dimensional structure of the corona and the large scale magnetic field reflected in

the structure of the heliosphere?

How, and with what time delay, are changes in the corona reflected in changes in heliospheric

conditions?

What ix the role of large scale coronal mass-ejection transients in the evolution of the corona, and

what ix their impact on heliospheric conditions?

With regard to the effect of the Sun on the Earth by modulation of heliospheric structure, what is the

relative importance of recurrent events such as the passage of coronal holes, and of more transient

occurrences such as those associated with coronal mass ejection events? Is there a significant

correlation between the conditions in the heliosphere and atmospheric phenomena on the Earth such

as weather, or ix heliospheric influence limited to phenomena such as the aurora?

2.5 Non Solar Observations with ASO: A potentially significant aspect of the ASO program is the possibility of

observing active phenomena in other stars with ASO. We plan to explore the scientific implications, and

engineering impact of such observations during the Concept Study.

2.6 A Detailed Scientific Program for the High Resolution Telescope Cluster: The scientific objectives

discussed above emphasize the broad nature of the problems the powerful combination of instruments that make up

the ASO can address. It is important to emphasize, however, that each ASO component ensemble is avcry powerful

and advanced observatory, which is capable of addressing a broad range of spccific and fundamental issues. Therefore,

at each stage of the evolutionary ASO development, the ASO will be capable of observations at the cutting edge of

astronomical research. The scientific program of OSL (formerly HRSO) has been well documented (4). Since the

High Resolution Telescope Cluster is expected to be the first of the ASO instrument ensembles discussed in this

proposal to be deployed, we present in Appendix A, a discussion of scientific objectives to be addressed with its high

resolution X-Ray. XUV. EUV. and UV instruments. Discussions of the scientific programs of P/OF and HEF are

contained in the conference proceedings edited by Tandberg-Hanssen. Wilson and Hudson (5b) and Chupp and Walker

(6).
REFERENCES

.

_2a°

2b.

o

4a.

4b.

4c.

5a.

5b.

6.

7.

8.

9a.

9b.

I0.

I1.

12.

13.

14a.

14b.

15.

16.

17.

18a.

18b.

19.

20.

21.

Astronomy and Astrophysics for the 1980s, Voi. I: Report of the Astronomy Survey Committee, (National

Academy Press, Washington, D.C., 1982.); An overview of the report of the full Survey Com-mittee is

contained in the article by George Field in Physics Today, 35, No. 4, 46, (April 1982.)

Challenges to Astronomy and Astrophysics: Working Documents of the Astronomy Survey Committee,

(National Academy Press, Washington, D.C., 1983.)

"Astronomy and Astrophysics for the 1980s, Voi. II: Reports of the panels," (National Academy Press,

Washington, D.C., 1983.); Overviews of the reports of the High Energy, Radio and Ultraviolet, Optical

and Infrared Panels are contained in a special issue of Physics Today, 35, No. 11, (November 1982.)

A.B.C. Walker, Jr., R. Moore, and W. Roberts, The Advanced Solar Observatory, NASA Technical

Publication (1986)

Stuart B. Jordan, "The Solar Optical Telescope (SOT)," Space Science Reviews, 29,. 333, (1981.)

Richard B. Dunn, "SOT Instrumentation." Ibid., 341.

Alan Title,'An Overview of the Orbiting Solar Laboratory," Lockheed Pal, Alto Research Laboratory

pre43rint (1988)

"The Pinhole/Occulter Executive Summary," NASA Techm'cal Paper 2089, (October !982.)

E. Tandberg-I-Ianssen, R.i_4. Wilson and A.S. Hudson,'Solar Flares and Coronal Physics; Using P/OF as a

Research Tool," NASA Conf. Pubi. 2421, (1986).

"On High-Energy Aspects of Solar Flares - A Basis for a High Energy Facility for Solar Physics", Special

Issue of the Journal Solar Physics; Eds. E.L,. Chupp and A.B.C. Walker, Jr., in press (1988)

W. Bailey, "Advanced Solar Observatory Acctmmodation Requirements Study," Teledyne Brown

Engineering, Inc., Huntsville, Alabama, Sept. 1988.

John Bahcali, "Solar Neutrinos: Theory versus Experiment," Space Science Reviews, 24, 277, (October

1979.)

Gordon Newkirk, Jr., and Kendrick Frazier,'The Solar Cycle," Physics Today, 35, No. 4, 25, (April 1982.)

Advances in Helio and Astmseismology, IAO Syrup. 123 Ed. J. Christensen, Dalogaard and S. Frandsen

(D. Reidel Publ. Co. Dodrecht, 1988)

Symposium on Seismology of the Sun and Sun-like Stars, Tenerife, Spain Sept. 1988 to be published by

ESA Publ. 286 Ed. E. J. Rolfe

R. Grant Athay and Oran R. White, "Chromospheric Oscillations Observed with OSO 8: IV. Power and

Phase SpeCtra for C II," The Astrophysical Journal, 229, 1147, (May 1979.)

Jack Harvey, "Observations of Small-Scale Photospheric Magnetic Fields," Highlights of Astronomy, 4,

223-239, (I 977.)

Jack B. Z_rker, Editor, "Coronal Holes and High Speed Wind Stream,." (Colorado Associated University

Press, 1977.)

Jack A. Eddy, "The Maunder Minimum," Science, 192, 1189, (June 1976.)

Gerard Van Hoven, "Plasma Energetics in Solar Flares," Highlights of Astronomy, $, 343, (1980.)

Reuven Ramaty and Richard E. Lingenfelter, "Gamma Ray Astronomy," Annual Review of Nuclear and

Particle Science, 32, 235, (1982.)

"Solar Physics: The Report of the Solar Physics Working Group of the Astronomy Survey Committee,"

Chapter 1 of Reference 2a (1982); An overview of the report of the Solar Physics Working Group is

contained in the article by A.B.C. Walker, Jr., in Physics Today, 35, No. 11, (November 1982.)

Physics of the Sun (3 Volumes), edited by P.A. Sturrock, T.E. Holzer, D.M. Mihalas, and R.K. Ulrich, (D.

Reidel Publishing Company, 1985.)

"Solar Variability, Weather, and Climate," (National Academy Press, Washington, D.C., 1982.)

O.C. Wilson, Arthur H. Vaughn, and D.M Mihalas, "The Activity Cycles of Stars," Scientific American,

224, 104. (February 1981.)

O.C. Wilson, "Chromospheric Variations in Main Sequence Star,." The Astrophysical Journal, 226, 379,

(December 1978.)

J. D. F. Bartoe and G. E. Bruckner, J. Opt. S,c. Am. 65, 13 (1975)

D. A. Schwartz et al, Proc. SPIE 597, 10 (1985)

T.W. Barbee, Jr., Optical Engineering 25, 898 (1986)
3.0 Instrumentation: The ASO will consist of 4 instrument clusters, the O_oiting Solar Laboratory (OSL), the

High Resolution Telescope Cluster (HRTC), the Pinhole Occulter Facility (P/OF), and the High Energy Facility

(HEF).The major components of the OSL, a 1.0 meter aperture visible light telescope (4) and a 0.3 meter ultraviolet

telescope, the High Resolution Telescope and Spectrograph (19) are well defined, and will not be addressed during the

present Concept Study (data on the OSL and HRTS are included in Appendix B, for the convenience of the reader).

The P/OF is the subject of a separate Concept Study proposal, and will also not be included in the present Concept

Study [The P/OF is shown in Figure 1lb, as it might appear mounted on the Space Station]. We will concentrate on

the High Resolution Telescope Cluster and the High Energy Facility in this Concept Study.

3.1 The High Resolution Telescope Cluster; The Advanced Solar Observatory High Resolution Telescope

Cluster (HRTC) is an array of telescopes that willprovide high resolution spectroheliograms in lines and continuum

emitted over the full range of temperatures (-2 x 104 to 5 x 107 K) present in the outer solar atmosphere with

angular resolution of--0.1 arc second. The HRTC (mounted on the Space Station Payload Pointing System (PPS) is

shown in Figure 1la). The full ASO HRTC consists of the following telescopes:

• A 40 - 50 can aperture ultraviolet telescope [U-l] covering the speclral range from 7.Z1200 - 3300 J_

• A 35 - 40 cm aperture EUV telescope [E-l] covering the spectral range Z_. 550 - 1200 J_

Three (3) or four (4) 35 - 40 cm aperture XUV telescopes [X-I to X-4] covering the spectral range

170 - 600 ,_

• A 35 - 40 cm aperture soft x-rayrXUV telescope [S-I] covering the spectral range _ 10 - 170 _

• A 35 - 40 cm aperture soft x-ray telescope [S-2] covering the spectral range X_. 1.5 - 25

• An X-Ray Flare Spectrometer [S-3] covering the spectral range 7,Z 1.5 - 150 J_

A 50 cm aperture Gamma Ray Imaging Device [T-l] covering the energy range from 20 keV to 1

MeV

A _uster of Global Dynamics Insmunents.

A possible arrangement of the major HRTC telescopes for the Space Station is shown in Figure 12. The

properties of the HRTC insmunents are summarized in Tables 2a and 2b.

Some of the ions whose principal lines will be observed by each telescope are shown in the footnotes to

Table la.

3.1.1 The Soft X-Ray, XUV, EUV and Ultraviolet Telescopes: The EUV and XUV strawman telescopes both

share the same optical design, a Ritchey-Chretien. Ray trace results have shown that on-axis resolution of 0.03 arc

seconds can be achieved in principal, and the resolution over a I0 min arcmin field is better than 0.I arcseconds. The

wavelengths imaged will be determined by the mirror coatings or surface composition, SiC for the EUV, and Si/Mo

multilayers for the XUV. The soft x-ray telescopes will both use the Wolter I configuration, but they will differ in

their grazing angle. We note that one possible development approach for the $2 Telescope is to utilize the AXAF

TMA mirror (20). This mirror has a focal length of -6 meters and an aperture of 40 cm, which are the same as the

proposed ASO $2 mirror. Slit-jaw images will be formed for all telescopes in H Ly-a, to provide information for

co-registration of images from each telescope, and for an aspect signal for pointing control and stability.

The EUV and XUV Telescopes: The basic configuration of the EUV and XU'V telescopes is illustrated

in Figure 13. The radiation from the primary mirror is shared by two focal planes, the first is illuminated by a Cassegrain/Ritchey-Chretien secondary which intercepts -90% of the light, and forms a focus behind the primary, at the

aperture of a tomidal grating spectrometer. The remaining 10% of the light is allowed to proceed toward the prime

focus where it is analyzed by a small Czerny-Tm'nermonochromater to produce a full disk monochromatic image.
angement of theASO HRTC Telescopes

Om_

M_ ........... ::::;;;;::..'E..'_'-.'_¢r:':_"

¢I

Figure 13. Optical layout of the EUV and XUV telescopes. M! and M2 are the mirrors of the Ritchey-Cretien telescope, S is

the aperture/slit of the teroidal grating speclmgraph, and F1 is the focal plane. Ct is the f'h-st Czerny-Turner collimating mirror

(it is nested in the center of M2), C/TG is a fiat grating, and C2 is the Czerny-Tumer focusing mirror. F2 is the wide field focal

plane. The scale is in centimeters. The envelope of E I. and XI-X4 is • cylinder 0.55 m in diameter and -7 m long. The

Czerny-Tumer grating and detector will Ixojoct beyond the envelope slightly.

_'_ WoOer

Mirror

Optical layout of the long wavelength Wolter Telescope, S-l.

Slit _ Ellipsoidal

Gratings, G1

Figure 14. The primary Wolter mirror is nested. S is the

entrance aperture to the Ellipsoidal Grating Spectromete¢, G 1 is an array of multilayer ellipsoidal grating, and F l is the focal

plane. M represents an array of multilayer tertiary mirrors which provide a wide field image at F2. The envelope for SI is a

cylinder 0.55 m in diameter and --7 m Ions. The ellipsoidal focal plane spectrometer will project beyond this envelope

slightly.

X'<_#csm sam

Co/utJ/walm, OC

Figure 15. Optical Layout of the short wavelength Wolter Telescope, S-2. OC are the objective double crystal arrays. FW is a

filter wheel, and F is the focal plane. The envelope for S-2 is • cylinder 0.55 m in diameter and -7.2 m long.
Table 2a.l_-openiesof theHRTC _menla

Sys_ Pmuy O_

I_ Umut

8*_ Amrlum F.L

cm m

SOectmscopicFocus WdeF_ldFocus

F.L Rxd Size F.L PixdSize Mkror

am pm am In Cea_q

Wde Feld

S_k_a_ Sl:ectogra_

U-v I_h_-C_akm 1_o.33mv 4o 5ooo

- ! Rik:_f*Ghmlm 550-I1002 40 $000

X-1 R_ch_*Chrmm 167-1773 40 $000

X-2 Ritchey.ClvaJm Ie7.2_04 40 sooo

X4 Ri_:hey-Chnam 24S- 2SSs 40 S000

x-4 Ri*ch_.Chr_ 27S- 2s6• 40 sooo

s-1 wdw I 10-17¥ 40 sooo

s-2 w,_,r i 1.s. 2s* 40 sooo

1-I Coklw SO 8ooo

S-3 NA. HA. N.A

I HI, CN.0Y,0VI

SI-b ILSiX.FeXII. F, XN

_0keV.1MaY

1.S-2Sl

2 Her Hd.UoX, OV.CIOVt

SF, XV.S_UOW

2o,ooo s (o.os-) sooo s (o.2,)

2o,ooos (o.os-) sooo s (o.2-)

2o,ooo s (o.o6-) sooo s to.z)

2o,ooo s (o.os-) sooo s (o.2"1

20,0oo s (o.os') sooo s (o.2"}

20,ooos (o.os-) sooo s (o.2")

_o,ooo s (o.lo') sooo s (o.2")

KA KA S000S (0.IS')

N.A. 6OOOgO (1.6")

N.A. HA. N.A HA.

3Fe_ FeX.OV, OVl

70 Vl. OVl, FeXlnI.FIXXI

AI TandemWadsw_h Czemy-Tmnw

('uA;. - s x vo_)

SiC TonxdalGrabn9 Czlmy-Tumer

OJA_,- S x 104)

Mo/Si TomidalGraling Czerny-Turne_

('u_). - 2 x Io4)

Ik)/si TomklalGmling Czemy-T.rner

0JA_. - 2 x 10_}

Mo/Si Torok_ Grat_j Czerny-Tumer

(_a_. - 2 x Io4)

ToroidalGming Czemy-Tumer

O,Ja_.- 2 x Io4)

All Elipsok_ Gralng Cassegnin

(_a_ -io4)

NX ('Us). -to4) N.A

4 _l _. r_l)Ol.FeXl_FeXXN

Tabk 2b Global Dynamics Instrumentation

nqSTRUMEHT

FOURIER TACHOMEI'FJU

MAGqETOGRAPH

DIFFERENTIAL

RADIOMETER

INTEGRATED LIGHT

SPF.CI'ROMETER

IIEI./OSEISMOMETER

SPECTRAl. APERTURE

RANGE "(cm)

3500-11.000 _ 30

4000-7000 A IS

4ooo.7oooA _o

40004000 A "i 2

RESOLVING IqJWER ..........

ANGULAR SPECTRAL TEMPORAL

(sre-tec/',cm_ (E/AE) (secon_ls_

0.5_$0 600,000 60

4/3O00 5 60

FULL SUN I 0 I

2/1500 ! 00,000 60

EXPLODED VIEW OF GRID TELESCOPE

tlpmmm

---, _' _'o..-."'-'"

Figure 16. An exploded view of GRID showing the und plates

holding the grid subcollimator pain and the Solar Disk Sextant

together with the cannister housing the detector modules.

Figure 1% A view of the High Energy Facility

mounted on the Space Station.

OPJGtNAL PAGE

BLACK AND _"HITE PHOTOGRAPH
The Soft X.Ray Telescopes: Two soft X-ray telescopes are employed. The first telescope (SI) is used

principally as a primary for multilayer secondary mirrors, which provide high resolution spectroheliograms. One

possible configuration is illusUated in Figure 14. (This "Wolter Cassegrain" configuration is discussed in Appendix

B.) The second telescope, $2 is placed behind an array of objective double crystal spectrometers (Appendix B), to

provide monochromatic images of high lemperamre active region and flare plasmas (Figure 15).

The Ultraviolet Telescope: The primary function of the Ultraviolet Telescope on HRTC is to provide

high resolution images which will allow the correlation of HRTC images with images obtained by OSL andP/OF.

These are several possible approaches to thedevelopment of the HRTC Ultraviolet Telescope, including (i) a

telescope/svectrometer identical to the EUV and XUV telescopes, (ii) an insu'ument employing the HRTS design of

Bartoe and Bruckner (19), (iii) a simple high resolution telescope employing interference filters only to achieve

spectral resolution. To some extent, the HRTC Ultraviolet Telescope configuration will depend on the configuration

of the ultraviolet telescope on OSL. Presently, the strawman OSL ultraviolet telescope utilizes the HRTS

configuration.

As Table 2a shows, the strawman x-ray, XUV, EUV and UV instruments are capable of achieving

resolution of 0.1 arc.u_ (0.05" pixels) over the wavelength range from 10 - 3000 A, and 0.2 arcsec resolution

between 1 and 10 A, satisfying a major,AgO objective.

3.1.1.0 Technological Issues: We brieflydiscoss a number of technological issues raised by the strawman designs

of the soft x-ray, XUV, EUV and UV telescopes, which we intend to address during ourConcept Study.

3.1.1.1 Multilayer Optics: Over the past decade, significant advances have occurred in the development of normal

incidence x-ray optics (21). These advances have resulted from the ability to fabricate thin film layered synthetic

microsu'uctures, "multilayers', with a level of reproducible AngstrOm-scale perfection which allows their use as

Bragg diffractors for soft x-ray and extreme ultraviolet radiation. Mulfilayer structures, when deposited Onsubstrates

of sufficient smoothness, can act as optical elements in high resolution telescopes. The resolution of the resulting

mirrors is determined solely by the optical figure of the substrata. Fabrication of these thin-f'dm layered synthetic

microstructures (multilayers) were pioneered by Eberhard Spiller at the IBM Watson Research Center (22a) and by

Troy W. Barbee (22b) at Stanford Uulversity. By depositing a series of alternating layers of two different materials,

one a strong x-ray scauerer with high atomic number, and the second a weak x-ray scatterer or "spacer" with low

atomic number, a synthetic structure that acts as an x-ray Bragg diffractor can be fabricated. X-ray reflectivity is

maximized when. the condition for constructive interference is satisfied as indicated by the Bragg equation, nX =

2deffsin0, where n is the order of diffraction, 0 is the angle of incidence with respect to the surface, deft is the

effective layer-pair thickness (corrected for refraction effects), and k is the wavelength of the incident radiation.

Although the reflectivity at each interface is small, by the use of tens to hundreds of layer pairs, excellent

reflectivities (>50% near normal-incidence) can be achieved. By proper selection of the materials and control of the

thicknesses of the scatterer and spacer layers, the multilayer mirrorcan be tailored to reflect a desired wavelength band

at maximum efficiency for a given angle of incidence. Multilayer structures can also be fabricated to have any layer

pair spacing between 15 and 500 A, thereby providing optical surfaces for radiation with 30<),<1000 A. Hence,

conventional asuonomical imaging/spectroscopic configurations employing normal-incidence reflection optics coated

with a multilayer of the appropriate layer pair spacing, can be made to operate in the soft x-ray wavelength regime

where in the past only grazing-incidence reflections could be used, and in the exu'eme ultraviolet, where in the past

only single reflection optical systems could be used. Normal-incidence x-ray optics have several distinct advantages

when compared with grazing-incidence optics: (i) for a given aperture the collecting area and field of view are

significantly larger;. (ii)compound normal-incidence opticalsystemscanbefoldedand thusbemade more compact;

(iii)normal-incidenceopticalsurfacesaresignificantly easierand cheapertomanufacture;and (iv)theaberrations

(resultingfrom violationsoftheAbb_ Sinecondition)associatedwithgrazing-incidence opticalandspectroscopic

systems can be avoided; (v) image degradation due to non-specular scattering is less severe. The spectral resolution of

a multilayer is determined by the number of layers that participate in the bragg-diffraction process, which in turn

depends on the total number of layers present and the amount of attenuation in each layer. For multilayer-coated

optics, the spectral bandpass (AMX) can be as large as 20% or as small as 1%. Normal-incidence reflection

efficiencies can be made as high as 70%.

The first successful imaging experiments with normal-incidence multilayer optics were carried out in the

laboratory during the early 1980's (23). Recently, Underwood et al. (24) have published a moderate resolution (- 10

arcsec) image of a single solar active region obtained with a single.reflection (Herschellian) multilayer telescope

having a bandpass centered at 44 A (corresponding to Si XII radiation). The pow.er of this new technology has now
been definitively demonswated by the results obtained by Walker et al (25) in a recent rocket-borne observation of the

Sun (front cover) using multilayers in the classic Cassegrain optical configuration. The success of conic multilayer

telescopes demonstrates that we are able to develop a single spectroheliograph instrument design which, with the

appropriate choice of mirror surface for each wavelength interval, can achieve re_.lution _3 -0.1" at ullraviolet

(-1000 - 2000 J_), EUV 000 - 1000 J_), XUV (100 - 300 _) and soft x-ray (30 - 100 A) wavelengths.

Barbee (26,27) has recently successfully fabricated and te.qed the lust multilayer gratings. Barbee has shown

that these combined microstructure optical elements disperse wavelengths Iragg-diffraeted by the muitilayer structure

so that all constructive interference occurs at essentially constant angles relative to the zero order bragg-diffracted

beam. The multilayers deposited on these gratings are Mo/Si layer pairs with deft -153 ,_. Measurements on these

gratings have demonstrated resolution as high as Z/A_.-2000 at normal incidence. These very exciting results

demonstrate that our strawman concept can provide the high resolution spectra required by ASO. Among the issues

related to multilayer optics we propose to investigate are the improvement of reflection efficiency and the

development of eHipsoidal and tcmidal multilayer gratings.

3.1.1.2 The Photoelectric Detector Arrays: A fundamental requirement for the development of a compact high

resolution EUV/XUV/soft x-ray speclroheliograph instrument is the availability of high resolutiondetector arrays.

The two most widely used imaging arrays are the Charged Coupled Detector (CCD) (28) and the Multi Anode

Microchannel Plate Array (MAMA) (29, 30). Both devices will be investigated. The MAMA detector has been

successfully used in ground based astronomical applications, and in space, and has been selected for the second

generation ultraviolet spectrometer (STIS) on Space Telescope, for two EUV instruments on SOHO, and for the

stellar EUV/XUV spectroscopic explc_.x mission, LYMAN. The CCD has been selected for Space Telescope and

AXAF, and has flown on several successful solar missions. The MAMA detector has the following advantages: (i) it

is flight proven, (ii) the MAMA can have extremely fast (in principle sub-microsecond) response time, which is well

matched to the dynamics of solar phenomena, (iii) the MAMA does not suffer from bias levels which vary across the

array (i.e.. non "flat field') as does the CCD, (iv) the nature of the MAMA (event detection by coincidence with

orthogonal anode arrays) allows compensation for image motion to be achieved, at the microsecond level, within the

detector, with virtually no compromise to the quality of the image [Iiling et al (31)], and (v) the MAMA can be

made with a photocathode that is blind to the visible, greatly simplifying the problem of suppression of scattered

light in UV, EUV, XUV and soft x-ray insmunenls. The advantages of the CCD include wide dynamic range and

availability in large size arrays (1000 x 1000). Three important issues related to photoelectric array detectors that we

plan to address are array size, pixel size, and dynamic rangr..

3.1.1.3 High Resolution XUV Sensitive Films: The image shown on the cover (25) was recorded on an EUV-soft

x-ray-sensitive, tabular grain (T-grain) emulsion (32) that was specially prepared by the Eastman Kodak Company.

This emulsion is similar to Kodak T-MAX 100, but was prepared without a gelatin overcoat for optimum

performance in the XUV. The film had a Rein-Jet backing which prevented static discharges during fdm transport in

the vacuum and was _ in conventional Canon 1"-70 cameras with no adverse effects. Previous observations

in the EUV/XUV region have been carried out with Schumann emulsions (33), which are extremely sensitive to

contaminants and abrasions and yield a maximum photographic diffuse density no greater than 1.6. The T-grain

emulsion yielded densities greater than 3.0 and high sensitivity throughout the spectral region studied during our

rocket flight (8 to 256 J_). We have extended our measurements with T-Grain 100 to 1200,_, using an EUV source at

Los Alamos Scientific Laboratories (34). Hoover et al. (35) have discussed the advantages of these new T-grain fdms

at soft x-ray-EUV wavelengths. More recently (34), we have carried out tests of a new XUV sensitive film based on

the very high resolution type 649 emulsion used in Kodak Specwo. scopic Plates. This new emulsion has been found

to have excellent sensitivity over the spectral range 40<Z<I600A. We will investigate the use of both the T-Grain

100 (resolution 200 l/mm) and the new 649 XUV sensitive emulsion (resolution 2000 l/ram) in a 70 mm format to

record HRTC observations. This will allow a field of 10' x 10' to be observed for the XUV and ELrv spectroscopic

foci, and full disk to be recorded for the wide field foci. No other presently available detector can approach film in

resolution (i.e., small pixel size), array size, or storage capacity. Two important issues which we plan to investigate

are photometric accuracy, and dynamic range of XUV dims.

3.1.1.4 Pointing, Aspect and Stability: The pointing accuracy (-60 arc sec - achieved by providing an error

signal from the HRTC solar sensor) and stability (+15 arc secYsec) possible with the PPS is not sufficient for the

HRTC. There are three possible approaches to stabilizing the images of the HRTC telescopes and spectrographs; (i)

the use of an active mirror servo for each telescope which is controlled by an error signal from a free sun sensor;, (ii)

the use of the image motion compensation capability of the MAMA detector which can correct the address of each

photon detected, based on an error signal JR. Uling, N. Zaum and R. Bybee (31)], or (iii) the use of a Fine Pointing

Gimbal (FPG), to stabilize each HRTC telescope. We have selected, as our baseline design, the use of active
U "

mirrors. The MAMA image motion compensation capability is very attractive, because of its simplicity (no moving

parts), however it cannot achieve stabilization of images recorded on film. The possibility of a FIG system is also

atwactive, and will be studied further, however, insufficient data on the characteristics of the Space Station PPS is

available to access this approach. One advantage of the active mirror approach is that Ball Aerospace has already

successfully demonstrated the stabilization of a solar telescope mirror to an accuracy of 0.08 arc sec with a handpass

of 600 Hz, and an amplitude of-,-J:15 arc min, using the Lockheed Rocket derived LISS fine Sun Sensor. Different

versions of this system have controlled 24", 6" and 2" _ to these specifications, which are adequate to stabilize

HRTC images. We propose to use an active mirror for each HRTC telescope to improve the pointing accuracy and

stability of the HRTC images to a fraction of an arc second. We also plan to develop a correlation tracking signal

from the ultraviolet telescope H Ly a image, which can be used to further improve the stability of the active

mirrors. The Bali Active Mirror Serves are described in Appendix C. We are confident that we can stabilize the

HRTC images to 0.05 arc sec, and possibly 0.02 arc sec.

3.1.2 Global Dynamics Instrumentation: The ASO strawman payload was developed before the SOHO

instruments were selected. The original ASO Global Dynamics Instrumentation (GDI) contained four instruments

(Table 2b).

• A Helioseismometer capable of precise measurements of the global "seismic" oscillations of the Sun, in

order to study the structure and dynamics of the convection zone and deeper interior.

• An IntegratedLightSpecu'ometercapableofobservingthefundamentalmodes ofthesolaroscillations.

• A magnetograph/tachometer capable of studying the large scale sumcture of the solar magnetic and velocity

fields.

• A differential radiometer capable of studying the dynamical behavior of convective cells on the Sun.

The first two requirements appear to be met by insmunents scheduled for deployment on SOHO. Our fLrsttask in

regard to the Global Dynamics Instrument package will be to determine which ASO requirements can be met by

SOHO instruments. The appropriate ASO GDI studieswill then be undertaken.

3.1.3 Hard X-Ray and Gamma Ray Imaging: Recently, Prince et al (36) have reviewed techniques for the

imaging of hard x:_ys and gamma rays from solar flares. The article by Prince et al provides a detailed review of the

technology of gamma my collimators and detectors; we refer the reader to this up-to-date and comprehensive review.

The GRID instrument under development for a MAX 90 balloon flight is compatible with the HRTC dimensions,

and we propose to evaluate the impact of its inclusion in HRTC. The characteristics of GRID are given in Table 3

below. The P/OF will, of course, be able to achieve higher angular resolution (-0.1 arc see) for hard x-rays.

Table 3 - Characteristics and capabilities of a GRID for use on a Balloon

Detector type Scintillation counters

Detector area 80 cm2

Energy range 20keV to I MeV

Subcollimator material Tungsten and tantalum

Number of subcollimators

Sensitivity

Angular resolution

Field of view

32

>I00 flares per week

1.6 arc sec

Full Sun

3.1.4 X-Ray Flare Spectroscopy: The X-Ray Polychromator (XRP) instrument on SMM (37) demonstrated the

importance ofhighly time resolvedhigh resolution x-ray spectra overa series ofwavelength bandscontaining the

He-like line mulfiplets of ions from Fe XXV (5 x 107 K) to O VII (2 x 106 K) for the study of flares. Observations

of this nature are highly complementary to the high spatial resolution narrow band observations of the HRTC soft xray, XUV, and EUV telescopes. These lines are emitted in the wavelength range k_. 1.5 - 25 J_ and offer many

diagnostic iines for density, temperature, and non-thermal excitation (38). The XRP utilized bent crystal

spectrometers to obtain these highly resolved spectra; and we plan to study a similar instrument for inclusion on

HRTC.
3.2 TheHighEnergyFacility:TheHighEnergyFacility(HEF)consi_of fivemajorspectrometers (TableI, p.

3), (i) high and (ii) low resolution Gamma Ray Line Spectrometers (I0 keV - I0 MeV), (iii) High Energy Gamma

Ray Line (I0 MeV - I00 MoV) and (iv) Neutron (I0 MeV - I000 MeV) specwometers, and (v) a Low Frequency

Radio Specuc_raph. The parameters of the HEF Gamma Ray and Neutron insmunems is given in Table 4.

Table 4

Parameters of the Four Instruments

in the High Energy Facility

X-ray and Gamma Ray Line Soecirometers ¢2_

Field of View:. Full Sun

Total Area: High-resolution; 540 cm 2

High sensitivity;, 2000 cm 2

Spectral Resolution: High resolution:;

High Sensitivity:;

Time Resolution : High resolution;

High Sensitivity;,

10 keV at 4.4 MeV

200 keV at 4.4 MeV

lsec

0.25 sec

I-Ii_hEnert_, Gamma Ray and Neutron S,ectrometers (2)

Field of View: Full sun

Total Area: High Entergy Gamma Ray;, lm 2

High Enrergy Neutron; lm 2

Time Resolution: 1 sec

Speclral Range: Gamma Ray Spectrometer 5 - 150 MeV

Neutron Spectrometer 30 - 1000 MeV

Spectral Resolution: High Energy Gamma Ray Spectrometer AE/E = 0.2 at 50 MeV

High Energy Neutron Spectrometer AE/E = 0.2 at 100 MeV

Weight

Power

Telemetry Rate

Volume

I-lEGS HENS _ High Sensitivity

1000 Kg 700 Kg 400 Kg 1000 Kg

40 W 40 W 40 watts 60 watts

5 Kbps 5 Kbps 80 ICos 120 Kbs

1 m3 1 m3 0.7 m diam x 0..5 m 1 m diam x 0.5 m length

The proceedings of the recent "Conference on the High-Energy Aspects of Solar Hares - a Basis for a High Energy

Facility for Solar Physics," are currently in press at Solar Physics (6). Contained in these proceedings are detailed

reviews on the properties of the High Energy Facility gamma ray and neutron spectrometers, including papers on

Hard X-Ray Gamma Ray Spectrometers (39). High Energy Gamma Ray Spectrometers (40), Polarization

Measurements (41). and Neutron spectrometers (42-44). Technological questions and alternative instrument designs

for HEF instruments are discussed in depth in these references, and will only be summarized here. The issues to be

addressed include: (i) properties and relative advantages of scintillation materials including NaI (TI), CsI (T1),CaI

(Na) and Bismuth Germanate or "BGO', ('d)the properties and relative advantages of semiconductor detector materials

including Silicon, Germanium, Mercuric Iodide, and Cadmium Teiluride, (iii) pulse shape discriminauon (to reduce

backgrounds) in semiconductor detectors, (iv) the design and construction of collimator grids for imaging

•instruments, and (v) imaging detectors for transform imaging instrument, including multiwire gas proportional

counters, gas drift chambers, liquid Argon and Xenon detectors, and inorganic scintillators.
4.0 Data Acquisition, Calibration and Archiving:

4.1 Analysis of ASO Data Acquisition Modes: A detailed analysb of the implications of the data acquisition

modes that may be employed by ASO will be undertaken. The modes to be studies are (i) compression of data from

photoelectric detector arrays, (ii) storage of data on broad Space Station on optical disks for transport via the Shuttle,

real time telemetry via TRSS, and (iii) the use of photographic f'dm. We plan to develop a detailed ASO data plan.

4.2 Data Reduction and Science Data Formats: This ASO data plan will def'me a standard data format with

calibration incorporated. These formats could be generated at SSL/MSFC. and provided to ASO users, and we will

investigate this possibility. The HRTC film will be microdensitometered at MSFC or at JSC.

4.3 Archiving of Data: We plan to develop a detailed plan to archive ASO reduced data in a standard format such as

the NOAO (IRAF) or the NRAO formats.

S.0 Orbiter Crew Requirements: The ASO requirements for the Orbiter Crew are (i) to replace f'dm cameras when

the film has been exposed and (ii) to provide commands to select the location on the sun to which the ASO is to be

pointed, (iii) to replace consumables such as cryogenic fluids or solids, and (iv) to replace defective equipment where

possible, as required. We anticipate requesting active crew control of the ASO pointing on rare occasions, for special

objectives such as flare observations. If a wide band link to ground is available on a sufficiently frequent basis, then

requirement (ii) above is not necessary. The crew must be trained to (a) remove and replace film cameras during

EVA, and Co) recognize and select solar features from the H Ly a telescope display, (c) to replace other consumables

(only optical disks and cryogens are identifiable at this time) and (d) to replace electronic modules.. Skill Co) may

not be required if wide band ground link is available on a sufficiently frequent basis.

6.0 Development of ASO Management and Cost Plans: We plan to carry out, at MSFC, a detailed engineering

analysis of the ASO, based on the work of the Instrument Scientists, and Mission Scientists, and on the engineering

analyses to be carried out by TRW, Ball Aerospace, and Brown Teledyne Engineering. Based on this analysis, we

will develop detailed management and cost plans for the ASO program.
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Appendix A

A Scientific Program for the ASO

High Resolution Telescope Cluster

° . .
Frontispiece II

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ORIGINAL PAGE IS

OF.. POOR QUALITY

(a) Model of chromospheric network, circa 1976.

(b) Model of chromosphericnetwork, circa I988.

Evolution of models of the Solar Atmosphere: Figure (a) is a model of the two dimensional structure of the

chromospheric (supergranulation)network pro.p__, by Gabriel in 1976 (Ref. 4 on p. 26), which was inspired by the

observation (Reeves, Ref. 3) that transition region (I0 _ K < T < 106) material is concentrated in the boundaries of

the network. Figure Co)is a more recent model proposed by Dowdy, Rabin, Moore, and Emslie (Rots 22-26) which

incorporates a more sophisticated representation of the chromospheric fine structure. It is clear that observations of

the quality planned for HRTC (--0.I arc sec - 70 kin) are essential to test models of chromospheric fine structure.
.

1.0 Introduction: A major objective of the High Resolution Telescope Cluster (HRTC) is to address fundamental

problems relating to the structure and phenomenology of the solar atmosphere, including (i) the chromospheric

network and magnetic fine structure, (ii) coronal loops, (iii) large scale coronal structure and dynamics, (iv) the

corona-solar wind interface, and (v) flares. We discuss specific objectives which can be addressed with the HRTC

instruments below. In the discussion, we will assume that the UV, XUV, and soft x-ray observations of the HRTC

will allow structures in the solar atmosphere covering the entire range of temperatures present in the chromosphere

and corona (104K - 5 x 107K) to be observed with very high spatial (-0.1") and spectral (Z/AZ >8000) resolution

simultaneously.

1.1 Chromospheric and Coronal Structure and Dynamics: In the past decade, observations made from rocket

and satellites at UV, EUV/XUV and soft/x-ray wavelengths have clearly demonstrated the fundamental role that

magnetic fields play in defining the structure and the flow of mass and energy in the chromosphere and corona, and

have resulted in the identit'r.ation of coronal holes as the source of high speed solar wind streams. Differences in the

physical conditions in the chromospheric and coronal Myers over different regions of the solar surface appear to be

intimately related to the configuration and strength of the underlying photospheric magnetic field. We may think of

the chromosphere and corona as divided into three kinds of regions, distinguished by their magnetic field structure;

(i) active regions, characterized by strong local photospheric and coronal fields with closed configurations; (ii) quiet

regions characterized by "chromospheric" network structure (see Frontispiece II) at transition region temperatures

(10 4 - 7 x 105 K) and weak coronal magnetic fields that appear to be closed on a large scale; and (iii) coronal holes,

associated with weak coronal magnetic fields having a diverging open field configuration. The regions with strong

fields are characterized by enhanced radiative output, and characteristic temperatures exceeding - 106 K. The x-ray

emission of active regions is dominated by plasma confined to magnetic flux tubes, i.e., coronal loops.Quiet regions

are characterized by average chromospheric radiative output and coronal temperatures near 106 K, although cooler

loops (-105 K) are also frequently observed. Coronal holes are characterized by somewhat lower chromospheric

radiative output, low coronal radiative output and densities, and characteristic coronal temperatures generally less than

106 IC Bright points (small bipolar regions with enhanced XUV and x-ray emission) are found in coronal holes,

however, and appear to be associated with streamers and polar plumes that are sources for the solar wind. The

differences in the physical conditions in these regions must be due in part to the ability of magnetic fields to channel

the flow of mass and energy in the corona by the formation of loops, and in part to the role of the solar wind as a

major energy loss mechanism in open field regions. Since the magnetic field plays a dominant role in the solar

atmosphere, w_ .m_t be able to resolve structures on scales at which the field changes significantly. For the solar

atmosphere, this scale is -100 - 200 km or less, corresponding to 0.15 - 0.30" when viewed from near-Earth orbit.

The HRTC will be able to achieve a resolution of .-0.10" (0.05" pixels) or better.

1.1.1 The Chromospheric Network and Magnetic Fine Structure: In his comprehensive review, Athay (1)

points out that the chromospheric network has been known since the late nineteenth century. SKYLAB observations

have demonstrated that network structure can be observed in lines emitted by plasmas at temperatures as high as

7 x 105 K [Reeves, (2)]. The chromospheric network is characterized by fine scale (-1.0" or less) structures (3, 4),

such as spicules (vertical magnetic flux tubes which contain cooler material and appear as dark fibers). The dynamic

nature of this fine structure is demonstrated by the observation of red shifted and blue shifted line profiles in material

associated with network structures such as spicules [Dere, Bartoe and Brueckner, (5a)], and small scale loops

(observed in lines such as C IV _. 1550 A corresponding to T - 105 K) which contain material at transition region

temperatures [Dere et al., (5b)] (see Frontispiece IF). Each of the network structures observed at chromospheric

temperatures (- l0 t K), such as spicules, appear to have counterparts visible at ultraviolet wavelengths

corresponding to emission by material at wansition region temperatures (-105K). In addition, distinct structures,

such as the larger scale "macrospicules" and C IV emitting loops are observed at ultraviolet wavelengths. Recent

Spacelab-2 observations (5c) suggest that the macrospicules, which show growth and decay on scales of minutes, are

composed of very small scale structures containing upward flowing material that may counter balance down flowing

material observed at lower resolution [see also Athay (1) p. 57ff]. The structure and dynamics of the network at

temperatures greater than 105 K is only now being explored at the level of 1 arc sec (6).

Two fundamental questions relating to the chromospheric network which can be addressed with the HRTC

are.:

(i) What is the relatio_hip of microflares, bright point$ and spicule$ to magnetic flux cancellation?
(ii) What ix the relationship of microflares to spicules and coronal heating?

An unsettled observational question basic to the solar cycle is that of Ihe removal of magnetic flux from the solar

atmosphere, the process that over the course of each cycle removes all of the flux that emerges from below the

photosphere. Sequences of photospheric magnetograms having nearly the best resolution that can be obtained from

the ground (about I axr.sec) have revealed examples of fine-scale merging and cancellation of positive and negative

clumps of flux. Simultaneous H-ct movies show that the flux cancellation was accompanied by microflare activity

in the chromosphere above [Martin, Livi, and Wang (7)]. These findings arc consistent with flux removal by a

combination of reconnection and submergence, but the present observations do not have enough resolution to prove

this [Rabin, Moore., and Hagyard (Sa), Zwann (9)]. If flux is in fact removed by submergence [Mnore and Rabin

(Sb)], tben the solar cycle might be a process of recycling old magnetic field rather than a process of generating new

magnetic field. On the other hand, if flux is primarily removed by eitha" upward extraction or resistive destruction of

the field [by for example, genc_rating bright points (10 a-c) and emphcrical bipolar active regions (10t')], then the

solar cycle hlay be mainly a process of generating new magnetic field rather than a recycling process. To decide

between these pivotal alunmatives for flux removal, we need subarcsecond resolution of the photosphere,

chromosphear¢, transition region, and corona simultaneously to see how flux is removed or destroyed. That is,

subarcsecond imaging simulL_neously in visible, ultraviolet, EUV, and X-ray or XUV radiation is required to show

the form of the associated f_-sr_lc activity in the chromosphere, transition region, and corona and its role in the

flux cancellation seen in the photosphere. The necessary observations of the photospheric magnetic field might be

obtained by new high-re, solution imaging techniques at the best ground-based sites, or from a magnetograph flown

on a high-altitude balloon or in space.. The HRTC will supply the necessary high-resolution imaging of the

chromosphere O--Ycx),_nsition region 0JV/EUV), and corona (XUV/Soft X-ray).

Images of the transition region and corona obtained from rockets, SKYLAB, and the Solar Maximum

Mission with spatial resolution in the range 1-10 arcsec, along with photospheric magnetograms and chromospheric

images from the ground, have dcmonstrat_ that the heating of the corona, transition region, and chromosphere is

directly tied to the magnetic field [e.g., see Parker (11)]. In active regions, studies of such data have shown that

heating is strongest where the field is most nonpotential, as evidenced by shear in the field across polarity inversion

lines [Orrail (12). Webb and Zirin (13). deJ.a_ach et al (14). Gary et al (15)]. This suggests that heating is

accomplished by some form of current dissipation. In any case, all fields in active regions are strongly

inhomogeneous on subarcsecond scales [Moore and Rabin (10). Zwann (9)]. so it is likely that the basic character of

the heating is l}i.dckm.below the resolution of present observations. One poss_ility, proposed by Parker (16a,b,c,d).

is that the heating is by microflaring in the fine-scale subsmicture of the field. Spatially unresolved microflaring has

been observed in UV emission from the transition region in active regions [Porler, Toomre, and Gebbie (17a), Porter

et al (17b)]. Apparently, simultaneotis subarcsecond resolution of the photosphere, chromosphere, transition region,

and corona is required to establish how the substructure in the field configurations with large-scale shear in active

regions is involved in heating. In quiet regions, heating is concentrated in the magnetic network, the location of

chromospheric spicules. It is plausible to suppose that spicules are basically smaller versions of macrospicules, the

surge-like eruptions known to be generated in microflares in the small magnetic bipoles that appear as bright points

in soft X-ray images of a few arcseconds resolution [Moore et al (18)]. Microflaring in UV emission from the

transition region has also been observed in tiny bipoles throughout the network [Porter et al , (19)]. These

observations raise the question of the role of microfiares and spicules in the heating of the chromosphere, transition

region, and corona (16a,b.c,d; 19; 20; 21) It will take subarcsecond resolution in visible or UV light to see the

structure of chromospheric spicules and how Ihey ate generated. It will lake subarcsecond resolution of the HRTC in

EUV and XUV or X-ray radiation to see ff and how microflares are important for spicule generation and/or coronal

heating.

A third question we wish to address with the HRTC is:

(iii) What ix the relationship between fine-scale magnetic structures (spicules. bright points, cool loops.

etc.) and heating of the quiet transition region and corona?

From; (i) observed structural similarities of the lower transition region (104 < T < 105 K) with the chromosphere,

(ii) the failure of coronal back-heating models of the transition region to provide enough plasma for the lower

transition region, and (iii) the fine-scale mixture o[ pol_lrities seen in the magnetic network in quiet regions, one is

led to the view that most of the plasma in the 10 -10" K temperature range in .the quiet solar atmosphere is not
maintainedbyheattransferfomthecorona[RabinandMoore(22),Dowdy,Rabin,and Moore (23)]. An alternative

picture, consistent with the observed fine-scale mixture of polarities in the network, is that most of this plasma is

contained in small magnetic loops in the network (4). and is thereby magnetically insulated from coronal backheating, and therefore must be heated internally (22,23). From a modeling analysis that was prompted by the

observation of enhanced emission from the lower transition region at places in active regions where vector

magnetograms show nonpotentiality, it has been shown that one attractive candidate for the internal heating of the

lower transition region, both in active regions and in quiet regions, is resistive heating by fine-scale electric currents

(14,22). It has also been pointed out that magnetograms of quiet regions imply that much of the magnetic flux

linking the corona with the transition region constricts (undergoes a reduction in area) by a factor of order 100 in

passing down through the transition region (23) (see the cartoon in Frontispiece II). Modeling analyses [Dowdy,

Moore, and Wu (24); Dowdy Emslie, and Moore (25); Dowdy (26)] of the effect of such a large constriction on

conducfive,,heat_qow and on the differential emission measure through the temperature range of the upper transition

region (10"-10" K) indicate; (i) that the conductive heat loss from the corona may be as much as an order of

magnitude less than in previous such modeh that assume much less constriction, and (il) that models with the larger

constriction demanded by high-resolution magnetograms have too little emission measure in both the upper and

lower transition region. This suggests;, (i) that substantially less heating is reqttired to sustain the corona than

previously thought (because conductive losses ate less), and (ii) that the upper transition region is similar to the

lower transition region in that most of the solar plasma in the temperature range of the upper transition region is

also heated internally rather than by heat transfer from the corona. To test the above ideas, we need to see, at

suharesecond resolution, the magnetic structure of the chromosphere, transition region, and low corona. Along with

photospheric magnetograms, this will show directly; (i) the spreading out of the field that reaches up through the

transition region into the corona and (ii) whether most of the solar plasma at transition region temperatures is

contained in the feet of coronal loops, or instead is contained in small closed loops that are thermally insulated from

the corona. The HRTC will have the necessary spatial resolution and spectral range to reveal these structures.

1.1.2 The Structure and Dynamics of Coronal Loops: A major objective of coronal studies is to significantly

improve the empirical models of density, composition, structure, and temperature in coronal loops, and to use these

improved models to study the energy balance, heating mechanisms and dynamical behavior of loops. The structure,

dynamics and energetics of coronal loops have been discussed by a number of authors (27 - 33). Three major

questions relating to coronal loops are:

(i) "W_t is the nature, magnitude, spatial and temporal profile of the energy input (or inputs) which is

required to establish and maintain the thermal structure of quasi.static coronal loops?

How are the heating profiles related to magnetic structure: specifically, are the regions of most

intense heating associated with neutral surfaces? Does the inhibition of conductive energy loss

processes by the magnetic configurations of loops have a significant effect on their structure?

(iii) What is the detailed physical structure of coronal loops (i.e. filling factor and density, nature and

location of thermal gradients, etc.)?

With regard to the question of the heating mechanism, there are three main classes of models that are

currently being considered, a dynamic flare-type process, wave heating, and DC current dissipation. Each of these

mechanisms has observable consequences that can be tested with the HRTC.

In the flare-type heating process the energy input to closed coronal loops is befieved to occur in discrete

impulsive events, probably related to flares. Micro-flares have been detected in both X-Rays [Lin et al, (34)] and the

ultraviolet [Porter et al, (35)]. If such events occur down to very small scales [Parker (16c)] then they could account

for the heating of active regions. With the HRTC active region loops can be observed from the transition region to

the hottest plasma with sufficient spatial (0.10 arcseconds) and temporal (<1.0 second) resolution to observe microflaring directly if it is present. A key feature of the HRTC is that by virtue of the high spectral resolution of its

component telescopes it will be able to obtain images of structures containing plasma in a very narrow temperature

range. This is crucial for observing the solar corona since the coronal plasma is optically thin, so that a given lineof-sight necessarily traverses many different structures. Without a very fine discrimination in temperature, it is not

possible to determine the true limit to the fine-scale structure of active region loops and, consequently, to the
variability of the plasma. This is a major problem with broad-band tiltergrams obtained with grazing incidence

telescopes such as the SKYLAB S-054 and S-056 instruments. With the HRTC, it will be possible to determine the

variability of plasma at a single temperature and, hence, to test critically the micro-flare heating model.

The other possibility for coronal heating is that it is a steady.state process. For this case two general types

of models have been proposed, one is that the heating is due to wave dissipation [e.g., lonson (36), Holiweg (37),

Davila (38)], the other is that it is due to the dissipation of DC currents [e.g. Parker (39), Sturrock and Uchida (40),

Van Ballegnoijen (41)]. Both models are similar in that the ultimate power source is the stressing of coronal

magnetic fields by photospheric motions, the key difference is in the time scale of the dominant motions and of the

coronal electric currents. Since the basic physics of the two processes are quite different, they predict a different

dependence of the heating rate on magnetic field strength and loop length. For example, in the DC model the heating

rate is expected to increase with decreasing loop length (40), whereas in the wave picture the heating is expected to

be maximum at some loop length that depends on the power spectrum of the photospheric motions and the Alfven

speed in the corona. With the HRTC, it will be possible to resolve active region loops and accurately measure their

lengths, to determine the differential emission measure along a loop, and to deduce the total radiative losses of the

plasma. This will allow us to obtain the coronal heating rates for loops of various lengths and field strengths.

Hence, we will have the necessary data for delermining the relation between heating and loop length, thereby testing

which of the two heating processes, ff either, is supported by observations.

The second major question on active regions that the HRTC will address concerns their structure. For the

high temperautre coronal plasma, T • 106 If., it is known that the dominant structure is that of a loop; however, the

structu_ of the transition region is still not known. Observations from SKYLAB [Feldman 42)] and HRTS [Dereet

al., (4)] indicate that the Iransition region does not consist of simply the footpoints of hot coronal loops, as in the

standard model, but these data do not have sufficient resolution to determine the swacture. Another difficulty with the

standard model for the transition region is that observed differential emission measure at these temperatures has long

been recognized as being incompatible with the predictions of coronal loop models. Several theoretical models have

been proposed to explain this discrepancy. Athay (43) has argued that the transition region emission originates from

heated spicule material, Rabin and Moore (22) argue that it is due to plasma heated by very fine scale electric

currents, and Antiochos and Noci (44a) argue for "cool loops'. The question of transition region structure is

important not only for understanding the solar corona but is also of great importance for stellar studies. In particular,

one explanation for the observation that very-late type giants do not exhibit coronal properties (the "coronal dividing

line') depends critically on whether the solar transition region is indeed due to cool loops [Antiochos, I-Iaisch and

Stern(44b)]. l_m.m the viewpoint of observations, the main distinguishing feature between the transition region

models is that they predict three very distinct morphologies. With the high spatial resolution of the HRTC, it will

be possible to prove or disprove all three models. Both spicules and cool loops will be fully resolvable at 0.10

areseconds. Hence, the HRTC should finally reveal the true structure of the transition region.

For the hot coronal plasma the main outstanding morphological questions are; (a) what is the filling factor

of the hot loops, (b) how are they distributed on the solar surface, and (c) what is their relation to the network? The

transition region emission, T - 105 K, is well-known to originate from, structures that are found predominantly in

the chromospheric (supergranular) network. This emission is also known to have a small filling factor. (5) On the

other hand the quiet coronal emission T = 106 K is believed to originate from loop structures that are unrelated to the

network and that have a filling factor of order unity. The morphology is not clearly known for the hot T • 2x 106 K

active region plasma. There is reason to believe (44a) that this material originates from low-lying loops which

should have a small Idling factor. In addition, the theories for coronal heating generally predict that magnetic-energy

dissipation occurs in thin regions which should have very small tilling factors. Hence, there appears to be

disagreement between the observed structure of the corona and the predicted structure. There are also discrepancies

between the structure observed for the plasma at different temperatures. Better resolved observations with high

temperature discrimination are clearly needed. With the HRTC it will be possible to observe at high resolution (0.1")

the corona at a variety of temperatures and to determine in detail the dependence of morphology on temperature. The

HRTC high spectral resolution observations will be especially important in determining electron density [using

diagnostic techniques (45)], from which the tilling factor can be inferred. Such results would answer the questions

raised above, and would be invaluable for furthering our understanding of both the magnetic structure of active

regions and the heating of the corona.

A primary HRTC objective is to define the pre-flare morphology of the vector magnetic fields of active

regions in the photosphere and their connectivity with coronal fields, and to follow the evolution of the photospheric

fields and see how the coronal fields respond. Specific questions to be addressed are:
Or) Can we identify dynamical processes in coronal loop structures which might be responsible "forthe

preflare brightenings observed at soft x-ray wavelengths? If such phenomena are observed, how

are they related to magnetic neutral surfaces and what is their role in the heating of coronal loops?

(v) " What istherelationshipbetweenthethermalstructureofcoronalbrightpointsand themagnetic

configuration (andpossiblemodesofmagneticdissipation) ofthecorona?

The HRTC will be a very powerful tool for the study of the correlation of nonpotenfial characteristics of the

magnetic field with enhanced coronal heating, and the relation between x-ray emission and the morphology of the

field. Using various computational models (see paragraph 1.1.3.1) we can extrapolate the measured photospheric field

upwards and compare the resulting coronal field models with the field inferred from observed x-ray structures. Vector

magnetograph observations show that the photospheric field in active regions exhibits nonpotential characteristics

even in regions that do not produce significant flare activity [Hagyard and Rabin (46)]. X-ray images of the corona

reveal enhanced emission associated with bipolar regions with strong fields [Walker et al. (6)]. These two

observational results suggest a study of the correlation between areas of enhanced x-ray heating and non-potentiality

of the underlying magnetic field. Three obvious objectives are: (i) investigating the relation between the intensity of

the x-ray emission and the degree of nonpotentiality of the field; (fi) determining the differences between nonpotential

field characteristics of regions that exhibit x-ray emission and those that do not; and (iii) looking for differences in

the magnetic field in regions with x-ray heating as opposed to regions that flare. Recanfly, Gary and Moore (47) have

drawn attention to a number of sunspots where part of the umbralregion is covered by bright plages and have related

this phenomenon to the morphology of the magnetic field. It seems that the marked division of plage intensity in

the umbrae correlates strongly with the direction of the photospheric field there. The correlation is in the sense that

the plage is associated with field lines that rise from the sunspot and fall into areas interior to the active region. The

physical explanation is 0rot the enhanced heat generated by magnetic reconnection of the strong fields interior to the

active region is conducted along the magnetic field into the umbra. Since there is no equiyalent heating source

exterior to the active region, where the fields are much weaker, no heat is conducted along the magnetic field coming

into the umbra from outside the region. Hence the sharp division of umbral emission in step with the change in

direction of the field lines. An obvious extension of this correlation is to investigate this "bifurcation" phenomenon

in the x-ray regime.

Investigatigns relating to dynamical processes and abundance anomalies in coronal lops will benefit from

the HRTC spectrograph observations of line profiles. Specific questions to be addressed include:

(vi) What istheroleofmass transportincoronalloops?Isthereevidenceforabundanceanomalies?Is

thereevidenceforabundancedifferencesbetweencoronalloopsand coronalholes?

1.1.3 Large Scale Coronal Structure and Dynamics: The large scale structure of the corona, is dominated by

the solar magnetic field, as the high resolution image of the corona in the light of the emission of Fe IX and Fe X at

-173 ,_ on the cover demonstrates. Multilayer optics have a very great advantage over grazing-incidence optics in the

study of large scale coronal structure because normal incidence telescopes can achieve high resolution (--0.1 - 0.3

axcsecond) over a field covering the entire solar disk. In contrast, grazing-incidence optics (48) have severe coma over

fields exceeding -1' - 2', resulting in degradation of their resolution to many arcseconds at the solar limb for a Wolter

I mirror pointed at sun center. We discuss the application of the high resolution spectroheliograph observations,

which the HRTC will obtain, to the study of the large scale structure and dynamics of the corona.

1.1.3.1 Large Scale Magnetic Structure: Since it is difficult to measure the coronal magnetic field directly at the

present time [see the discussion of coronal magnetic field determination by Walker (49)], it is generally necessary to

calculate the coronal field from the boundary conditions provided by the measured vector field in the photosphere,

subject to reasonable assumptions and constraints (50). In order to guide these calculations, it is desirable to compare

the calculated field structure with observed coronal structures such as loops, filaments, the magnetic network,

streamers, etc. Clearly, high resolution images such as the one shown on the cover, covering the full range of

temperatures present in the atmosphere, will be a very valuable resource for the study of large scale coronal magnetic

structure and dynamics. Using multithermal observations we will address the following question:
(i) What is the large scale structure of the coronal field, and how is it restructured by events such as

flares and coronal mass ejections (CME's)?

High-resolution x-my images provide information on the morphology of the coronal field but little

quantitative data can be derived from these observations. Generally. the quantitative information is derived by

extrapolating upwards the measured photospheric field and comparing the computed coronal field morphology with

the x-ray observations. The extrapolations can be done assuming either potential or force-free configurations for the

field. The single fine-of-sight component of the field is sufficient for the potential calculation and for the "constant -

alpha" force-free case, but only if the region is observed near disk center. To extrapolate the field under the

generalized force-free assumption and to correctly extrapolate the field for the potential or constant-alpha force-free

cases for off-disk center regions, all three components of the magnetic field must be known. At MSFC, for example.

the observational program with the solar vector magnetograph is supported and enhanced by a number of numerical

and theoretical models and computer codes to extrapolate the photospheric field up into the corona. The the threedimensional configuration of the magnetic field above active regions can be analyzed using any of five computational

approaches: (i) the Schmidt potential calculation (51). ii) the Teuber potential method (52). iii) the NakagawaRaadu-Wellck linear force-free calculation (53). iv) Alissandrakis' finesr force-free method (54), and v) Wu's nonlinear

force-free calculation (55). In collaboration with Pridmore-Brown MSFC scientists are developing a second method

for the nonlinear force-free problem, a variational Galerkin technique (56). To conect for off-disk center effects,

MSFC scientists have developed computer programs to u'ansform the measured vector magnetic field and imageplane coordinates into heliographic coordinates. The HRTC images will greatly enhance this type of analysis.

1.1.3.2: Coronal Dynamics: Over the past decade, accumulating knowledge Shows that the sun's corona is a

dynamical, interwoven, magnetic structure with rapid energy releases occurring through its volume, producing mass

eruptions and million-degree temperature plasma. Coronal holes undergo continuous evolution which releases high

speed solar wind flows into heliospheric space. It is evident from theoretical studies that the dynamical behavior of

the corona is caused by the deformation of the magnetic field due to convective motion at the photosphere. The

precise nature of these magnetic deformations and the resulting MHD waves is crucial to the understanding of the

physics of these dynamical activities. These activities are also responsible for the general phenomena of solar and

stellar x-ray emissions, winds, flaxes, and eruptive mass ejections. In order to meet the challenge of the understanding

of physical processes involved in these observational phenomena. S.T. Wu has developed a number of timedependent, multi-dimensional, non-linear magnetohydrodynamic (MHD) numerical models. These models can be

formulated in either Cartesian, cylindrical or spherical coordinate allowing the analysis of observed phenomena in

various geomeeries_ and have been successfully applied to the study of a number of solar phenomena. For example,

Wu and his collaborators have investigated photospheric shear motions in active regions. This work has improved

the understanding of the mechanism for the storage of energy in the sheared magnetic field [Wu, et al.. (57a, b)] and

of the energy releases associated with eruptive phenomena [Wu, et al..(58)]. Wu et al. (59) have also studied

photospheric converging and diverging motion in reference to the neutral line to understand the emerging and

submerging magnetic flux. We plan to extend these studies of corona/dynamics using HRTC data. and to study

large scale coronal heating and solar wind acceleration by searching for evidence of MIlD waves propagation through

the photosphere and chromosphere. The HRTC spectroscopic observations of line profiles will be important for

this last objective.

Since the discovery of coronal mass ejections (CMEs) by the coronagraphs on hoard the SKYLAB and

SOLWIND P-78 experiments, CME's have been recognized as a major coronal phenomena; their study is essential

for the understanding of coronal physics. Many fundamental questions pertaining to the initiation, acceleration and

propulsion mechanisms of CMEs remain to be resolved. One main reason these fundamental questions have not been

resolved is that there are no direct observations which follow ejected material from the sun's surface up through the

corona with a single instrument. During the SKYLAB and SMM eras, attempts were made to carry out coordinated

campaigns by using multiple instruments, however there were always key observational parameters missing. The

HRTC instrument has the capability to observe CMEs from the sun's surface up through the corona. Because of this

unique capability, we can address directly the following questions related to CME's:

(ii) What mechanisms are responsible for the initiation, acceleration and propulsion of CAfEs?

(iii) What are the manifestations of CME's in the corona and solar wind and their associations and

correlations with solar and interplanetary events?
k. •

(iv) What ix the role of the CMEs in entrained magnetic fields?

(v) What is the role of CAfE's in the evolution of coronal holes?

For the study of coronal transients in general

and CMEs in particular, a number of authors have

investigated the coronal dynamical response due to a

pressure pulse which models observed flaring density

and temperature profiles; the calculated coronal

responses do resemble observed features of coronal

transients (c.f. CMEs) [Nakagawa et al ( 60, 61);

Dryer et ai.(62); Steinolfson, et a1.(63); Sime and

Hundhausen, (64); Wu et el., (56; 66; 67)]. A

schematic description of the._ models in relation to

their physical regions of applicability is shown in

Figure I. Employing these models as a diagnostic

tool to study the physical mechanisms involved in

the phenomena observed by HRTC will allow

ob_rved parameters to be used as a guide to refine the

physical models.

Figure 1 Schematic representation of physical region

where v&ious models apply.

1.1.3.3 Prominences and Filaments: In both active regions and quiet regions, magnetic neutral fines across

which there is strong magnetic shear are often marked by filaments (traditionally observed in absorption on the disk

in lines such as H-a) of chromospheric material suspended in the low corona by the sheared field (68). The same

structures can be observed as prominences in emission on the limb both in H a, and in high resolution XUV images

(6,69), There ia.a .close association between filaments and the eruption of flares in nearby active region loops (49);

the f'dament serves as a field tracer showing that the sheared field in and around the filament erupts explosively in the

impulsive phase of a flare. Quiet-region filaments mark neutral lines between opposite-polarity remnants of decayed

active regions. These filaments often erupt at the onset of a two-ribbon flare and are ejected from the Sun at the core

of a coronal mass ejection. Hence, it is of great interest in connection with flares and coronal mass ejections to

determine the magnetic field configuration in and around quiet-region f'daments, how this configuration forms and

evolves, and how it changes when the sheared field holding the filament erupts. The interface between filaments,

which ate embedded in the corona, and hot u'ansition region _d coronal gas is also a subject of considerable interest.

The filament channels that are seen in the cover image, appear to be due in at least part to absorption (6); however,

suppression of coronal emission by the magnetic structure of the filament may be important as well. Among the

questions involving filaments and prominences which can be effectively addressed by HRTC observations ate:

(vO Does the field configuration in and around quiet-region filaments have a different topology than

that of active-region filaments?

(vii) Are quiet-region filament eruptions basically flares; i.e., are they just slow versions of massejection flares in active regions?

(viii) Is fiux cancellation at the photospheric neutral line an important process in forming the magnetic

field configuration of quiet-region filaments?

1.1.3.4 Streamers and Plumes: In coronal holes, the loss of energy and mass to the solar wind becomes an

important element in the energy balance of the atmosphere. Withbro¢ (70) has discussed the structure and dynamics

of coronal holes, and the need for high resolution observations to address questions relative to the acceleration and

OF POOR QUALrI'_
heating of the solar wind. Figure 2 (70) illustrates the strong dependence of line width in the solar wind on the

nature of the heating mechanisms which operate in the solar wind. The HRTC spectroscopic observations will

allow line profiles to be studied in polar plumes and sueamers. Since bright points are observed near the base of

polar plumes, it has been suggested (71) that bright points may be the source of polar plumes and streamers. To

address these fundamental questions on the nature of the solar wind/corona interface, we will require observations

which combine high spatial resolution, high sensitivity, high spectral resolution (to observe line profiles), and good

thermal discrimination. The cover illustrates the power of multilayer telescopes, such as those under study for

HRTC, for the study of the solar wind/corona interface.

Figure2: Spectrallinewidthsplottedas • functionof

distancefrom Sun-center. The solidcurve is for

Lyman-cx widthscalculatedfora simpletwo-fluidmodel

withoutany Alfvenwave flux;thedashed lineisfora

model with Alfven wave fluxadded to account more

satisfactorilyfor the empirical Lyman-ct widths

measured (points) in a polar region where • coronal

hole was located. From the results of the two-fluid

model calculations one can estimate the behavior of •

line from • heavy ion (e.g., Fe XII Z 1242). The

figure shows how the width of this line would vary in

• model with Alfven waves present (the narrow crosshatched region) and without Al[ven waves (the nearly

horizontal cross-hatched area, which reflects

uncertainties in the amount of electron/Fe XII and

proton/Fe XII coupling). The empirical Fe XII width

(point near r = 1743) is typical of measurements made in

quiet coronal regions. Similar behavior is expected for

XUV lines such as Fe XII _. 192J[. HRTC will obtain

line pmf'des for both Fe XII Z 192 and k 1242.

LC i.; 2.¢ 3.0 4,0 g.O S._ io.o

, Ill.}

A funda,me.ntal question related to streamers and plumes that can be addressed by the HRTC is:

(ix) What is the thermal and density structure of coronal streamers and polar plumes, and what is their

relationship to coronal bright points? What is their contribution to the solar wind?

1.1.3.5 Coronal Holes and the Solar Wind: Observations of the evolution of the boundaries of coronal holes ate

especiaUy pertinent to the large-scale reconfiguration of the corona. X-ray images reveal the actual structures forming

a hole boundary. Observations of transition region lines such as He II (3. 256 or 304 A) reveal the boundaries of

coronal holes in the transition region. Coronal holes have been established as the sources of high speed wind

streams; the polar plumes observed in the polar coronal holes in white light images are thought to represent the flow

of materialinto the solar wind.

The scientific goals of the HRTC related to Coronal Holes and the Solar Wind are: (i) to explore the

thermodynamic structure of the corona; (ii) to relate the structure and evolution of the corona observed in the

EUV wavelengths to the global density structure; (iii) to investigate the regions in which the solar wind originates

and is accelerated; (iv) to extend the knowledge of the solar corona to the interpretation of integrated flux

measurements of stellar coronae; (v) to investigate the influence of the total coronal flux on the earth and its

plasma environment. A general discussion of the solar wind can be found in Pnenman and Orrall (72a).

Thermodynamic Structure: The design of the HRTC allows it to be sensitive to lines and to line

multiplets arising in plasmas over a range of temperatures between about 104K and 5 x 107K. As a result, the

global distribution of emission measure observed can be used to infer the temperature in a variety of coronal and

chromospheric structures in a variety of conditions. The high resolution of the instrument coupled with its wide field

of view will thus allow small scale structures to be studied in the context of their wider surroundings and the global

organization of the corona. In particular, the relationship of small structures, at a variety of temperatures, to the

global density structure will allow constraints to be developed on the physical precesses by which the large scale

ORIGINAL "_ _ _"

OF POOR QUALITY
structurecan be supported.High temperaturefinesareavailablewhich willbe used to identifyflaresand otheractive

phenomena. This willpermit investigationof the relationshipof thesephenomena to theglobalcoronalstructure

along with their influence on the organization of the corona and the solar wind and its heating. A specific example

will be the investigation of any temperature signature in the corona associated with mass ejections which might

reveal compact regions of heating before, during or after the eruptions.

Coronal Structure and Evolution : The temporal resolution of the HRTC will allow the evolution of

the coronal temperature distributions to be followed, both on the long time scale (weeks to years) associated with the

gradual evolution of the corona, and also on the rapid scales (minutes and tens of minutes) associated with flares,

with prominence eruptions, and with mass ejections. This will provide information on the durations over which

energy build up and dissipation occur for a variety of scale sires and will indicate the time scales over which various

physical processes are important.

Of particular interest will be the identification of the features on the solar disc which can reveal the

development and eruption of mass ejections. This will allow the inference of imminent interplanetary disruptions in

the vicinity of the earth, and will be an important step to overcoming the current restriction with conventional

coronagraphs that coronal activity can only be observed above the limb. As a result, we can reasonably expect to

understand more clearly the signature of such mass ejections in the interplanetary medium. This will in turn provide

stronger constraints on the mechanisms by which mass ejections are formed and the physics of their propagation

through the interplanetary medium. Further, by establishing a history of disc structures as they rotate towards the

limb, we will have a knowledge of the development of any structure which does develop to become a mass ejection

at the limb. This too will serve to place valuable conswaints on candidate mechanisms for CME production.

The interaction of the long evolutionary scale and the short impulsive activity time scale in the corona will

be studied with special interest. The details of this interaction should reveal whether mass ejections cause or result

from the long term evolution of the coronal density and temperature distributions, or perhaps neither.

Solar Wind: The ability to infer coronal temperature in a variety of coronal structures, such as streamers

and polar plumes, will permit us to investigate the physical properties of the regions in which the solar wind

originates (72a) and is accelerated (72b). A specific example is in the investigation of the properties of spicules,

which can be investigated both as to their global distribution and their detailed structure with HRTC. Knowledge of

their distribution and physical properties compared with the relationship to the global organization of the corona

should provide information to investigate more fully their role in solar wind generation. Similarly, by providing

temperature estimates within coronal holes the observations from HRTC will impose constraints on a number of

theories of the. h_ting of the corona and solar wind (72b). Further, the high resolution for the instrument will

minimize the effect of uncertain flow tube geometry, a feature which serves to increase uncertainty in the comparison

of many theories with observations.

Since SKYLAB, it has been suggested that there may be an x-ray brightness discontinuity inside coronal

holes. Some streamline geometries have been theoretically predicted to result in standing, reverse shock waves

within 0.5 solar radii of the photosphere (73). This would be most likely to occur in transient coronal holes with

large geometrical divergence. The very high sensitivity and dynamic range of the multilayer telescopes in the HRTC

baseline design will be well suited to a search for x-rays emitted from the heated plasma downstream of these shocks

- addressing the questions of their existence and conditions of formation.

1.1.4 Flares: Walker (49) has recently reviewed the requirements for multispectral flare observations. The

requirements for flare observations at XUV/EUV and soft x-ray wavelengths are summarized by Walker in a series of

objectives, related to each of the five phases of the flare event.We review these requirements, and the objectives of

the HRTC related to flares studies.

1.1.4.1 The Preflare State: The basic mechanism of a solar flare must involve the storage of energy in the coronal

magnetic field [Spicer, Mariska and Boris, (74)]. Such field configurations are generally accepted to be force-free and

to contain in situ electric currents [Van Hoven et al. (75)]. However, there are few direct measurements of transition

region or coronal fields, and there is only indirect evidence for the existence of coronal currents. It is imperative that

a knowledge of the development of the coronal magnetic field configurations which precede the impulsive phase of a

flare be obtained. We may state this objective as follows:
(i) The dynamics and evolution of the coronal magnetic field structures associated with the flare site

must be determined in sufficient detail and with sufficient precision to determine the location and

magnitude of storedenergy,and the location,source,and effectof associatedcurrentflows.

Observations with the MSFC vector magnetograph have shown that, for 4 cases studied so far, flares start

above the photosphere at a place where the local photospheric magnetic field has a maximum 'shear' in its azimuth

[Hagyard and Smith 76)]. By shear, we mean that the direction of the transverse component of the magnetic field is

aligned neatly parallel to the magnetic neutral line rather than perpendicular to it as would be the case if the field

were in a potential configuration. This coincidence raises some questions that are basic to understanding the flare

process. First, how is this sheared configuration generated, how is it connected to the coronal fields where the flare

instability occurs, and how do these coronal fields respond as the shear increa._? Second,,dnes the flare process

begin as the result of a further increase in shear, presumably beyond some "critical" value, and what are the critical

parameters in the coronal field? Or does the flare erupt as the result of external perth, such as reconnection of

• the flux. or submergence of flux, all of which can lead to significant changes in coronal topology in response to the

formation of current sheets induced by relatively minor changes in the photosphere? Third, does the field relax from a

sheared to a more potential configuration as the result of the flare, and where does this relaxation occur7.

To address these questions we need a knowledge of the solar magnetic field at all levels of the solar

atmosphere. However, reliable measurements can only be obtained in the photosphere and chromosphere. We

propose to infer coronal fmlds from such observations, using the HRTC XUV coronal images (see Soc 1.1.2, p.5).

Knowledge of the physical parameters governing the preflare plasma is critical to an understanding of the

conditions which must precede the energy release which signals the impulsive phase of a flare. Many of the

questions which are raised by the population of non-thermal particles accelerated during the impulsive phase, such as

their source, the anomalous abundances observed, and their energy spectrum ['Forman, Ramaty and Zweibel (77)]

may depend critically on the configuration of the preflare plasma. We may state, therefore, a second objective:

(ii) The preflare dynamics and evolution of the solar plasma in the vicinity of the acceleration region

must be observed in sufficient detail to allow the detailed parameters [density, temperature,

departure (if any) from thermal equilibrium, composition of the material which is accelerated] to

be determined.

We k_ow fro m observations of magnetically confined coronal loops [Webb, (78); Withbroe, (79)] that the

thermal gradients within these loops must occur on scales smaller than several thousand kilometers (2 - 5 arcseconds

as viewed from the earth). In terms of the scientific problem we wish to address, a goal of observations of diagnostic

quality (Le. isolating lines excited over a nan'ow temperature range), with angular resolution -0.1 arcseconds appears

to be justLqed, and can be attained by the proposed HRTC strawman instruments.

1.1.4.2 Release of the Stored Energy: Kahler et at. (80) have argued that the mechanism which causes the

release of the stored flare energy must be due to a rapid change in the currents which generate the magnetic field

structure of the preflare region. These currents may be within the volume where particle acceleration occurs, or they

may be elsewhere. Kahler et aL further argue that collision-dominated reconnection is the microscopic mechanism by

which current energy is dissipated into heat. Spicer, Mariska and Boris (74) suggest that the trigger mechanism may

be the non-linear excitation of the ion-acoustic instability, and resulting anomalous dissipation. Assuming that the

flare energy is indeed stored in situ in coronal loops, we would expect that the occurrence of the flare trigger

mechanism will be accompanied by rapid changes in currents within the loop, and rapid heating of the gas confined

in the loop. We might expect the gas to manifest non-thermal ionization structure and electron velocity

distributions. Thus, measurements of the type discussed above (Objectives i and ii) would appear to be appropriate,

however temporal resolution on the order of seconds or less would appear to be necessary to follow the course of the

trigger mechanism. The question of the scale on which the trigger mechanism operates has not been satisfactorily

resolved theoreticaUy. Estimates as small as I km or less have been discussed (74); in most cases trigger mechanism

scales are expected to be no larger than 10 - 100 km. Clearly, very high resolution is essential if this aspect of the

flare phenomena is to be observed in sufficient detail to allow comparison with theory. We may state the objective

related to the flare trigger mechanism as follows.
(iii) The expected rapid change in current density that initiates the release of the stored magnetic

energy causing the flare, and the anticipated heating and possible non-thermal changes in the

plasma, must be observed directly on the appropriate temporal and spatial scales, which appear to

be seconds and 100"s of kilometers or less.

Once again, the HRTC can provide the necessary observations.

1.1.4.3 The Impulsive Phase: The rapid variation in emission from small scale structures which characterizes the

impulsive phase places the most severe requiteznents on multispectral observations of this phase of the flare. For

example, Canfield and Gayley (81) have shown that the profile of H-ct (and of course this applies to Ly ct also) is

sensitive to alternative models of impulsive phase energy transport, provided sufficiently high spectral (;UA_. -

20,000), temporal (- 0.05 seconds), and spatial (- 0.1 arcsecond or 70 kin) resolution can be achieved. Similarly,

very rapid, (< 0.1 seconds), but quite distinct changes in the plasma which emits the impulsive EUV radiation are

predicted by models based on heating by conduction or by non-thermal electron beams [Emslie and Nagai (82)]. In

order to observe the evaporation of this material, resolution of at least MZ_. - 104 is essential. Spatial scales should

be similar to those for the H-or emission. High spectral resolution observations of the prompt soft x-ray emission

have proved extremely useful in studying the heating and evaporation of chromospheric plasma to form the hot

postflare loop. Electron density and deviations Of the electron energy distribution from thermal equilibrium can be

deduced from the rados of satellite lines excited by inner shell excitation and dielectronic recombination [Fawcett et

al., Bely-Dubau et al. (83)], provided spectral resolution of MA_, - 104 is achieved. However, the spatial resolution

so far obtained is inadequate. There are indications of time structure on scales of -1 - 5 seconds, and more rapid

changes may be observable on smaller spatial scale, s. Very rapid motion [Antonucci et al. (84)] has been observed for

the x-ray emitting plasma in the impulsive phase, but the scale of these motions has not been resolved. Spadal

resolution sufficient to resolve loop structures (-0.3 arcseconds) is fiecessaty to understand the heating and

evaporation of the hot flare plasma. We may state the objective of the muldthermal impulsive phase observations as

follows:

(iv) The testing of models of the impulsive phase of a flare requires simultaneous multithermal

observations in visible, ultraviolet, EUVIXUV, and soft x-ray emission lines and continua, at

wavelengths corresponding to several thermal levels in each wavelength regime. Temporal

resolution must be at least on a scale of 0.I second for the EUVIXUV and ultraviolet. Spectral

resolution must be sufficient to measure line profiles, and spatial resolution should be at the 100

kilometer level on the sun.

The HRTC can achieve both the spectral and spatial resolution requited.

1.1.4.4 The Gradual Phase: A major question to be addressed in the gradual phase is the nature, duration and

location of the gradual energy release mechanism. Determination of the detailed thermal, spatial and dynamical

evolution of the soft x-ray emitting plasma in the postflare loop is a key observational goal [Strong et al. (85)]. The

presence or absence of non-thermal electrons during this phase is also a major issue. A major objective of the study

of the gradual phase is therefore:

(v) The flare generated thermal plasma must be observed with sufficiently high spatial (- 0.3

arcseconds) and spectral resolution to allow a detailed model of its evolution to be constructed.

Diagnostic measurements which are sensitive to non-thermal particle distributions are important

during this phase.

The HRTC is especially well suited to obtain the necessary observations, by virtue of the multithermal

observations made possible by the combination of grazing incidence and normal incidence Multilayer and SiC

Telescopes.

1.1.4.5 Atmospheric Restructuring: Itis important to study the restructuring of the atmosphere at wavelengths

sensitivc to all tcmpcrature levels present in the solar atmosphere. Among the phenomena of interest are filaments

(H-ct or H Ly-ct), chromospheric structure fLy-a, He II, C IV), the transition region (EUV/XUV) and the corona
(soft x-rays). Of special inwaest are Coronal Mass Ejections (CME's), and large scale hot coronal arches [Hildner et

at. (86)]. A serious deficiency in current studies of CME events is the gap between the level at which coronagraph

white light images can be obtained (-I..5 RO for SMlV0 and the height to which images of moving x-ray loops and

cool filaments can be followed using non-occulted telescopes. In the case of the coronal arches, higher spatial and

spectral resolution soft x-ray observations than were possible with the Hard X-Ray Imaging Spectrometer on SMM

arc required. The high resolution multithennal HRTC images will constitute an ideal data set for the study of these

post flare phenomena. Major objectives of the study of postflare atmospheric restructuring are as follows:

(vi) The study of the dynamic behavior of CME events, starting in the low corona and continuing to

several A.U. above the limb, is an important goal, as is the detailed study of the formation of

posCTare filaments and arches and the return of the solar transition region, chromosphere,

photosphere, and magnetic field structure to their normal state.

1.1.4.6 Discussion: Observations of flare phenomena clearly require simultaneous high angular resolution

observations of the solar magnetic field and the solar plasma covering a broad range of temperature (104 < T < 108

K') or excitation (for non-thermal phenomena). In most cases, high spectral and temporal resolution is also essential.

The HRTC will achieve 0.1 arcsecond resolution, and thermal coverage from 104 K (I-I Ly-a) to 5 x 107 (Fe XXVI

Ly-a). In addition, the Grating Spectrographs which are part of the HRTC can resolve line profiles, allowing

powerful thermal and dynamical diagnostic observations.

°
°.

f ;

22a.

22b.

24.

25.

26.

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Report Documentation Page

1. Report No.

4. Title and Subtitle

2. Government Accession No.

Advanced Solar

Accommodations

Final Report

7. Author(s}

Observatory (ASO)

Requirements Study

9. Performing Organization Name and Address

Teledyne Brown Engineering

300 Sparkman Drive NW

P.O. Box 070007

I untsville, AL 35807-7007

12. Sponsoring Agency Name and Address I

I George C. Marshall Space Flight Center

National Aeronautics and Space Administration

I arshall Space Flight Center, AL 35812

15. Supplemonmw Not_

3. Recipient's Catalog No.

5. Report Date

May 1989

6. Performing Organization Code

8. Performing Organization Report No.

10. Work Unit No.

11. Contract or Grant No.

NAS8-37128

13, Type of Repo_ and Period Covered

Final

14. Sponsoring Agency Coda

16. Abstract

This report presents the results of an accommodations analysis for the Advanced

Solar Observatory on Space Station Freedom. Concepts for the High Resolution

Telescope Cluster, Pinhole/Occulter Facility, and High Energy Cluster were

developed which can be accommodated on Space Station Freedom. It is shown that

workable accommodations concepts are possible. Areas of emphasis for the next

stage of engineering development are identified.

17. Key Words (Sugg_t_ by Author(s))

Advanced Solar Observatory, Solar

Physics, Sun, Space Station, HRTC,

P/OF

18. O_tribution Statement

Unclassified - Unlimited

19. Security Classif. (of thin report)

Unclassified

_. S_unW Clalmif. (of th_ _)

Unclassified

21. No. of _ges _. Price

426 A19"

f

NASA FORM 1626 OCT 86 *For sale by the National Technical Information Service,

Springfield, VA 22161-2171